Image decoding method and apparatus based on affine motion prediction in image coding system

ABSTRACT

An image decoding method performed by a decoding apparatus includes the steps of: acquiring motion prediction information on a current block from a bitstream; generating an affine MVP candidate list including affine motion vector predictor candidates for the current block; deriving CPMVPs for CPs of the current block on the basis of one affine MVP candidate among the affine MVP candidates included in the affine MVP candidate list; deriving CPMVDs for the CPs of the current block on the basis of the motion prediction information; deriving CPMVs for the CPs of the current block on the basis of the CPMVPs and the CPMVDs; and deriving prediction samples for the current block on the basis of the CPMVs.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(e), this application is a continuation ofInternational Application PCT/KR2019/008416, with an internationalfiling date of Jul. 9, 2019, which claims the benefit of U.S.Provisional Applications No. 62/698,001 filed on Jul. 13, 2018, No.62/703,415 filed on Jul. 25, 2018, the contents of which are all herebyincorporated by reference herein in their entirety.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The present disclosure relates to an image coding technology and, mostparticularly, to an image decoding method and apparatus based on affinemotion prediction in an image coding system.

Related Art

Demand for high-resolution, high-quality images such as High Definition(HD) images and Ultra High Definition (UHD) images have been increasingin various fields. As the image data has high resolution and highquality, the amount of information or bits to be transmitted increasesrelative to the legacy image data. Therefore, when image data istransmitted using a medium such as a conventional wired/wirelessbroadband line or image data is stored using an existing storage medium,the transmission cost and the storage cost thereof are increased.

Accordingly, there is a need for a highly efficient image compressiontechnique for effectively transmitting, storing, and reproducinginformation of high resolution and high quality images.

SUMMARY OF THE DISCLOSURE Technical Objects

A technical object of the present disclosure is to provide a method andapparatus that can enhance image coding efficiency.

Another technical object of the present disclosure is to derive anaffine MVP candidate list of a current block based on neighboring blocksto which affine prediction is applied and, then, to provide an imagedecoding method and apparatus performing prediction for the currentblock based on the derived affine MVP candidate list.

Another technical object of the present disclosure is to derive anaffine MVP candidate list of a current block by deriving a first affineMVP candidate list based on a left block group and by deriving a secondaffine MVP candidate list based on a top block group and, then, toprovide an image decoding method and apparatus performing prediction forthe current block based on the derived affine MVP candidate list.

Technical Solutions

According to an embodiment of the present disclosure, provided herein isa video decoding method performed by a decoding apparatus. The methodincludes the steps of obtaining motion prediction information for acurrent block from a bitstream, constructing an affine motion vectorpredictor (MVP) candidate list including affine MVP candidates for thecurrent block, deriving control point motion vector predictors (CPMVPs)for control points (CPs) of the current block based on one of the affineMVP candidates included in the affine MVP candidate list, derivingcontrol point motion vector differences (CPMVDs) for the CPs of thecurrent block based on the motion prediction information, derivingcontrol point motion vectors (CPMVs) for the CPs of the current blockbased on the CPMVPs and the CPMVDs, deriving prediction samples for thecurrent block based on the CPMVs, and generating a reconstructed picturefor the current block based on the derived prediction samples, whereinthe affine MVP candidates include a first affine MVP candidate and asecond affine MVP candidate, wherein the first affine MVP candidate isderived based on a first block in a left block group including abottom-left corner neighboring block and a left neighboring block,wherein the first block is coded with an affine motion model and areference picture of the first block is same as a reference picture ofthe current block, wherein the second affine MVP candidate is derivedbased on a second block in a top block group including a top-rightcorner neighboring block, a top neighboring block, and a top-left cornerneighboring block, and wherein the second block is coded with the affinemotion model and a reference picture of the second block is same as thereference picture of the current block.

According to another embodiment of the present disclosure, providedherein is a decoding apparatus performing a video decoding method. Thedecoding apparatus includes an entropy encoder obtaining motionprediction information for a current block from a bitstream, a predictorconstructing an affine motion vector predictor (MVP) candidate listincluding affine MVP candidates for the current block, deriving controlpoint motion vector predictors (CPMVPs) for control points (CPs) of thecurrent block based on one of the affine MVP candidates included in theaffine MVP candidate list, deriving control point motion vectordifferences (CPMVDs) for the CPs of the current block based on themotion prediction information, deriving control point motion vectors(CPMVs) for the CPs of the current block based on the CPMVPs and theCPMVDs, and deriving prediction samples for the current block based onthe CPMVs, and an adder generating a reconstructed picture for thecurrent block based on the derived prediction samples, wherein theaffine MVP candidates include a first affine MVP candidate and a secondaffine MVP candidate, wherein the first affine MVP candidate is derivedbased on a first block in a left block group including a bottom-leftcorner neighboring block and a left neighboring block, wherein the firstblock is coded with an affine motion model and a reference picture ofthe first block is same as a reference picture of the current block,wherein the second affine MVP candidate is derived based on a secondblock in a top block group including a top-right corner neighboringblock, a top neighboring block, and a top-left corner neighboring block,and wherein the second block is coded with the affine motion model and areference picture of the second block is same as the reference pictureof the current block.

According to another embodiment of the present disclosure, providedherein is a video encoding method performed by an encoding apparatus.The method includes the steps of constructing an affine motion vectorpredictor (MVP) candidate list including affine MVP candidates for acurrent block, deriving control point motion vector predictors (CPMVPs)for control points (CPs) of the current block based on one of the affineMVP candidates included in the affine MVP candidate list, derivingcontrol point motion vectors (CPMVs) for the CPs of the current block,deriving control point motion vector differences (CPMVDs) for the CPs ofthe current block based on the CPMVPs and the CPMVs, and encoding motionprediction information including information on the CPMVDs, wherein theaffine MVP candidates include a first affine MVP candidate and a secondaffine MVP candidate, wherein the first affine MVP candidate is derivedbased on a first block in a left block group including a bottom-leftcorner neighboring block and a left neighboring block, wherein the firstblock is coded with an affine motion model and a reference picture ofthe first block is same as a reference picture of the current block,wherein the second affine MVP candidate is derived based on a secondblock in a top block group including a top-right corner neighboringblock, a top neighboring block, and a top-left corner neighboring block,and wherein the second block is coded with the affine motion model and areference picture of the second block is same as the reference pictureof the current block.

According to another embodiment of the present disclosure, providedherein is an encoding apparatus performing a video encoding method. Theencoding apparatus includes a predictor constructing an affine motionvector predictor (MVP) candidate list including affine MVP candidatesfor a current block, deriving control point motion vector predictors(CPMVPs) for control points (CPs) of the current block based on one ofthe affine MVP candidates included in the affine MVP candidate list, andderiving control point motion vectors (CPMVs) for the CPs of the currentblock, a subtractor deriving control point motion vector differences(CPMVDs) for the CPs of the current block based on the CPMVPs and theCPMVs, and an entropy encoder encoding motion prediction informationincluding information on the CPMVDs, wherein the affine MVP candidatesinclude a first affine MVP candidate and a second affine MVP candidate,wherein the first affine MVP candidate is derived based on a first blockin a left block group including a bottom-left corner neighboring blockand a left neighboring block, wherein the first block is coded with anaffine motion model and a reference picture of the first block is sameas a reference picture of the current block, wherein the second affineMVP candidate is derived based on a second block in a top block groupincluding a top-right corner neighboring block, a top neighboring block,and a top-left corner neighboring block, and wherein the second block iscoded with the affine motion model and a reference picture of the secondblock is same as the reference picture of the current block.

EFFECTS OF THE DISCLOSURE

According to the present disclosure, an overall image/video compressionefficiency may be enhanced.

According to the present disclosure, efficiency in image coding based onaffine motion prediction may be enhanced.

According to the present disclosure, when deriving an affine MVPcandidate list, neighboring blocks are divided into a left block groupand a top block group, and an affine MVP candidate list may beconstructed by deriving MVP candidates from each block group. Thus, thecomplexity in the process of constructing the affine MVP candidate listmay be reduced, and the coding efficiency may be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a configuration of a videoencoding apparatus to which the present disclosure is applicable.

FIG. 2 is a schematic diagram illustrating a configuration of a videodecoding apparatus to which the present disclosure is applicable.

FIG. 3 illustrates a motion expressed through an affine motion model.

FIG. 4 illustrates the affine motion model in which motion vectors for 3control points are used.

FIG. 5 illustrates an affine motion model in which motion vectors for 2control points are used.

FIG. 6 illustrates a method of deriving a motion vector on a sub-blockbasis based on the affine motion model.

FIG. 7 is a flowchart illustrating an affine motion prediction methodaccording to an embodiment of the present disclosure.

FIG. 8 is a diagram for describing a method for deriving a motion vectorpredictor at a control point according to an embodiment of the presentdisclosure.

FIG. 9 is a diagram for describing a method for deriving a motion vectorpredictor at a control point according to an embodiment of the presentdisclosure.

FIG. 10 illustrates an example of affine prediction being performed in acase where neighboring block A is selected as an affine merge candidate.

FIG. 11 illustrates exemplary neighboring blocks for deriving inheritedaffine candidates.

FIG. 12 illustrates exemplary spatial candidates for constructed affinecandidate.

FIG. 13 illustrates an example for constructing an affine MVP list.

FIG. 14 illustrates an example for constructing an affine MVP list.

FIG. 15 illustrates an example for constructing an affine MVP list.

FIG. 16 illustrates a general view of an image encoding method performedby an encoding apparatus according to the present disclosure.

FIG. 17 illustrates a general view of an encoding apparatus performingan image encoding method according to the present disclosure.

FIG. 18 illustrates a general view of an image decoding method performedby a decoding apparatus according to the present disclosure.

FIG. 19 illustrates a general view of a decoding apparatus performing animage decoding method according to the present disclosure.

FIG. 20 illustrates a content streaming system structure to which thepresent disclosure is applied.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present disclosure may be modified in various forms, and specificembodiments thereof will be described and illustrated in the drawings.However, the embodiments are not intended for limiting the disclosure.The terms used in the following description are used to merely describespecific embodiments, but are not intended to limit the disclosure. Anexpression of a singular number includes an expression of the pluralnumber, so long as it is clearly read differently. The terms such as“include” and “have” are intended to indicate that features, numbers,steps, operations, elements, components, or combinations thereof used inthe following description exist and it should be thus understood thatthe possibility of existence or addition of one or more differentfeatures, numbers, steps, operations, elements, components, orcombinations thereof is not excluded.

Meanwhile, elements in the drawings described in the disclosure areindependently drawn for the purpose of convenience for explanation ofdifferent specific functions, and do not mean that the elements areembodied by independent hardware or independent software. For example,two or more elements of the elements may be combined to form a singleelement, or one element may be divided into plural elements. Theembodiments in which the elements are combined and/or divided belong tothe disclosure without departing from the concept of the disclosure.

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings. In addition, likereference numerals are used to indicate like elements throughout thedrawings, and the same descriptions on the like elements will beomitted.

Meanwhile, the present disclosure relates to video/image coding. Forexample, the method(s)/embodiment(s) disclosed in the present disclosuremay be applied to a method disclosed in a versatile video coding (VVC)standard or a next generation video/image coding standard.

In the present specification, generally a picture means a unitrepresenting an image at a specific time, a slice is a unit constitutinga part of the picture. One picture may be composed of plural slices, andthe terms of a picture and a slice may be mixed with each other asoccasion demands.

A pixel or a pel may mean a minimum unit constituting one picture (orimage). Further, a “sample” may be used as a term corresponding to apixel. The sample may generally represent a pixel or a value of a pixel,may represent only a pixel (a pixel value) of a luma component, and mayrepresent only a pixel (a pixel value) of a chroma component.

A unit indicates a basic unit of image processing. The unit may includeat least one of a specific area and information related to the area.Optionally, the unit may be mixed with terms such as a block, an area,or the like. In a typical case, an M×N block may represent a set ofsamples or transform coefficients arranged in M columns and N rows.

FIG. 1 is a schematic diagram illustrating a configuration of a videoencoding device to which the present disclosure is applicable.

Referring to FIG. 1, a video encoding device (100) may include a picturepartitioner (105), a predictor (110), a residual processor (120), anentropy encoder (130), an adder (140), a filter (150), and a memory(160). The residual processor (120) may include a subtractor (121), atransformer (122), a quantizer (123), a re-arranger (124), a dequantizer(125), an inverse transformer (126).

The picture partitioner (105) may split an input picture into at leastone processing unit.

In an example, the processing unit may be referred to as a coding unit(CU). In this case, the coding unit may be recursively split from thelargest coding unit (LCU) according to a quad-tree binary-tree (QTBT)structure. For example, one coding unit may be split into a plurality ofcoding units of a deeper depth based on a quadtree structure and/or abinary tree structure. In this case, for example, the quad treestructure may be first applied and the binary tree structure may beapplied later. Alternatively, the binary tree structure may be appliedfirst. The coding procedure according to the present disclosure may beperformed based on a final coding unit which is not split any further.In this case, the largest coding unit may be used as the final codingunit based on coding efficiency, or the like, depending on imagecharacteristics, or the coding unit may be recursively split into codingunits of a lower depth as necessary and a coding unit having an optimalsize may be used as the final coding unit. Here, the coding proceduremay include a procedure such as prediction, transformation, andreconstruction, which will be described later.

In another example, the processing unit may include a coding unit (CU)prediction unit (PU), or a transform unit (TU). The coding unit may besplit from the largest coding unit (LCU) into coding units of a deeperdepth according to the quad tree structure. In this case, the largestcoding unit may be directly used as the final coding unit based on thecoding efficiency, or the like, depending on the image characteristics,or the coding unit may be recursively split into coding units of adeeper depth as necessary and a coding unit having an optimal size maybe used as a final coding unit. When the smallest coding unit (SCU) isset, the coding unit may not be split into coding units smaller than thesmallest coding unit. Here, the final coding unit refers to a codingunit which is partitioned or split to a prediction unit or a transformunit. The prediction unit is a unit which is partitioned from a codingunit, and may be a unit of sample prediction. Here, the prediction unitmay be divided into sub-blocks. The transform unit may be divided fromthe coding unit according to the quad-tree structure and may be a unitfor deriving a transform coefficient and/or a unit for deriving aresidual signal from the transform coefficient. Hereinafter, the codingunit may be referred to as a coding block (CB), the prediction unit maybe referred to as a prediction block (PB), and the transform unit may bereferred to as a transform block (TB). The prediction block orprediction unit may refer to a specific area in the form of a block in apicture and include an array of prediction samples. Also, the transformblock or transform unit may refer to a specific area in the form of ablock in a picture and include the transform coefficient or an array ofresidual samples.

The predictor (110) may perform prediction on a processing target block(hereinafter, a current block), and may generate a predicted blockincluding prediction samples for the current block. A unit of predictionperformed in the predictor (110) may be a coding block, or may be atransform block, or may be a prediction block.

The predictor (110) may determine whether intra-prediction is applied orinter-prediction is applied to the current block. For example, thepredictor (110) may determine whether the intra-prediction or theinter-prediction is applied in unit of CU.

In case of the intra-prediction, the predictor (110) may derive aprediction sample for the current block based on a reference sampleoutside the current block in a picture to which the current blockbelongs (hereinafter, a current picture). In this case, the predictor(110) may derive the prediction sample based on an average orinterpolation of neighboring reference samples of the current block(case (i)), or may derive the prediction sample based on a referencesample existing in a specific (prediction) direction as to a predictionsample among the neighboring reference samples of the current block(case (ii)). The case (i) may be called a non-directional mode or anon-angular mode, and the case (ii) may be called a directional mode oran angular mode. In the intra-prediction, prediction modes may includeas an example 33 directional modes and at least two non-directionalmodes. The non-directional modes may include DC mode and planar mode.The predictor (110) may determine the prediction mode to be applied tothe current block by using the prediction mode applied to theneighboring block.

In case of the inter-prediction, the predictor (110) may derive theprediction sample for the current block based on a sample specified by amotion vector on a reference picture. The predictor (110) may derive theprediction sample for the current block by applying any one of a skipmode, a merge mode, and a motion vector prediction (MVP) mode. In caseof the skip mode and the merge mode, the predictor (110) may use motioninformation of the neighboring block as motion information of thecurrent block. In case of the skip mode, unlike in the merge mode, adifference (residual) between the prediction sample and an originalsample is not transmitted. In case of the MVP mode, a motion vector ofthe neighboring block is used as a motion vector predictor and thus isused as a motion vector predictor of the current block to derive amotion vector of the current block.

In case of the inter-prediction, the neighboring block may include aspatial neighboring block existing in the current picture and a temporalneighboring block existing in the reference picture. The referencepicture including the temporal neighboring block may also be called acollocated picture (colPic). Motion information may include the motionvector and a reference picture index. Information such as predictionmode information and motion information may be (entropy) encoded, andthen output as a form of a bitstream.

When motion information of a temporal neighboring block is used in theskip mode and the merge mode, a highest picture in a reference picturelist may be used as a reference picture. Reference pictures included inthe reference picture list may be aligned based on a picture order count(POC) difference between a current picture and a corresponding referencepicture. A POC corresponds to a display order and can be discriminatedfrom a coding order.

The subtractor (121) generates a residual sample which is a differencebetween an original sample and a prediction sample. If the skip mode isapplied, the residual sample may not be generated as described above.

The transformer (122) transforms residual samples in units of atransform block to generate a transform coefficient. The transformer(122) may perform transformation based on the size of a correspondingtransform block and a prediction mode applied to a coding block orprediction block spatially overlapping with the transform block. Forexample, residual samples can be transformed using discrete sinetransform (DST) transform kernel if intra-prediction is applied to thecoding block or the prediction block overlapping with the transformblock and the transform block is a 4×4 residual array and is transformedusing discrete cosine transform (DCT) transform kernel in other cases.

The quantizer (123) may quantize the transform coefficients to generatequantized transform coefficients.

The re-arranger (124) rearranges quantized transform coefficients. There-arranger (124) may rearrange the quantized transform coefficients inthe form of a block into a one-dimensional vector through a coefficientscanning method. Although the re-arranger (124) is described as aseparate component, the re-arranger (124) may be a part of the quantizer(123).

The entropy encoder (130) may perform entropy-encoding on the quantizedtransform coefficients. The entropy encoding may include an encodingmethod, for example, an exponential Golomb, a context-adaptive variablelength coding (CAVLC), a context-adaptive binary arithmetic coding(CABAC), or the like. The entropy encoder (130) may perform encodingtogether or separately on information (e.g., a syntax element value orthe like) required for video reconstruction in addition to the quantizedtransform coefficients. The entropy-encoded information may betransmitted or stored in unit of a network abstraction layer (NAL) in abitstream form.

The dequantizer (125) dequantizes values (transform coefficients)quantized by the quantizer (123) and the inverse transformer (126)inversely transforms values dequantized by the dequantizer (125) togenerate a residual sample.

The adder (140) adds a residual sample to a prediction sample toreconstruct a picture. The residual sample may be added to theprediction sample in units of a block to generate a reconstructed block.Although the adder (140) is described as a separate component, the adder(140) may be a part of the predictor (110). Meanwhile, the adder (140)may be referred to as a reconstructor or reconstructed block generator.

The filter (150) may apply deblocking filtering and/or a sample adaptiveoffset to the reconstructed picture. Artifacts at a block boundary inthe reconstructed picture or distortion in quantization can be correctedthrough deblocking filtering and/or sample adaptive offset. Sampleadaptive offset may be applied in units of a sample after deblockingfiltering is completed. The filter (150) may apply an adaptive loopfilter (ALF) to the reconstructed picture. The ALF may be applied to thereconstructed picture to which deblocking filtering and/or sampleadaptive offset has been applied.

The memory (160) may store a reconstructed picture (decoded picture) orinformation necessary for encoding/decoding. Here, the reconstructedpicture may be the reconstructed picture filtered by the filter (150).The stored reconstructed picture may be used as a reference picture for(inter) prediction of other pictures. For example, the memory (160) maystore (reference) pictures used for inter-prediction. Here, picturesused for inter-prediction may be designated according to a referencepicture set or a reference picture list.

FIG. 2 is a schematic diagram illustrating a configuration of a videodecoding device to which the present disclosure is applicable.

Referring to FIG. 2, a video decoding device (200) may include anentropy decoder (210), a residual processor (220), a predictor (230), anadder (240), a filter (250), and a memory (260). The residual processor(220) may include a re-arranger (221), a dequantizer (222), an inversetransformer (223).

When a bitstream including video information is input, the videodecoding device (200) may reconstruct a video in relation to a processby which video information is processed in the video encoding device.

For example, the video decoding device (200) may perform video decodingusing a processing unit applied in the video encoding device. Thus, theprocessing unit block of video decoding may be, for example, a codingunit and, in another example, a coding unit, a prediction unit or atransform unit. The coding unit may be split from the largest codingunit according to the quad tree structure and/or the binary treestructure.

A prediction unit and a transform unit may be further used in somecases, and in this case, the prediction block is a block derived orpartitioned from the coding unit and may be a unit of sample prediction.Here, the prediction unit may be divided into sub-blocks. The transformunit may be split from the coding unit according to the quad treestructure and may be a unit that derives a transform coefficient or aunit that derives a residual signal from the transform coefficient.

The entropy decoder (210) may parse the bitstream to output informationrequired for video reconstruction or picture reconstruction. Forexample, the entropy decoder (210) may decode information in thebitstream based on a coding method such as exponential Golomb encoding,CAVLC, CABAC, or the like, and may output a value of a syntax elementrequired for video reconstruction and a quantized value of a transformcoefficient regarding a residual.

More specifically, a CABAC entropy decoding method can receive a bincorresponding to each syntax element in a bitstream, determine a contextmodel using decoding target syntax element information and decodinginformation of neighboring and decoding target blocks or information ofsymbol/bin decoded in a previous step, predict bin generationprobability according to the determined context model and performarithmetic decoding of the bin to generate a symbol corresponding toeach syntax element value. Here, the CABAC entropy decoding method canupdate the context model using information of a symbol/bin decoded for acontext model of the next symbol/bin after determination of the contextmodel.

Information on prediction among information decoded in the entropydecoder (210) may be provided to the predictor (230) and residualvalues, that is, quantized transform coefficients, on which entropydecoding has been performed by the entropy decoder (210) may be input tothe re-arranger (221).

The re-arranger (221) may rearrange the quantized transform coefficientsinto a two-dimensional block form. The re-arranger (221) may performrearrangement corresponding to coefficient scanning performed by theencoding device. Although the re-arranger (221) is described as aseparate component, the re-arranger (221) may be a part of thedequantizer (222).

The dequantizer (222) may de-quantize the quantized transformcoefficients based on a (de)quantization parameter to output a transformcoefficient. In this case, information for deriving a quantizationparameter may be signaled from the encoding device.

The inverse transformer (223) may inverse-transform the transformcoefficients to derive residual samples.

The predictor (230) may perform prediction on a current block, and maygenerate a predicted block including prediction samples for the currentblock. A unit of prediction performed in the predictor (230) may be acoding block or may be a transform block or may be a prediction block.

The predictor (230) may determine whether to apply intra-prediction orinter-prediction based on information on a prediction. In this case, aunit for determining which one will be used between the intra-predictionand the inter-prediction may be different from a unit for generating aprediction sample. In addition, a unit for generating the predictionsample may also be different in the inter-prediction and theintra-prediction. For example, which one will be applied between theinter-prediction and the intra-prediction may be determined in unit ofCU. Further, for example, in the inter-prediction, the prediction samplemay be generated by determining the prediction mode in unit of PU, andin the intra-prediction, the prediction sample may be generated in unitof TU by determining the prediction mode in unit of PU.

In case of the intra-prediction, the predictor (230) may derive aprediction sample for a current block based on a neighboring referencesample in a current picture. The predictor (230) may derive theprediction sample for the current block by applying a directional modeor a non-directional mode based on the neighboring reference sample ofthe current block. In this case, a prediction mode to be applied to thecurrent block may be determined by using an intra-prediction mode of aneighboring block.

In the case of inter-prediction, the predictor (230) may derive aprediction sample for a current block based on a sample specified in areference picture according to a motion vector. The predictor (230) mayderive the prediction sample for the current block using one of the skipmode, the merge mode and the MVP mode. Here, motion information requiredfor inter-prediction of the current block provided by the video encodingdevice, for example, a motion vector and information on a referencepicture index may be obtained or derived based on the information onprediction.

In the skip mode and the merge mode, motion information of a neighboringblock may be used as motion information of the current block. Here, theneighboring block may include a spatial neighboring block and a temporalneighboring block.

The predictor (230) may construct a merge candidate list using motioninformation of available neighboring blocks and use informationindicated by a merge index on the merge candidate list as a motionvector of the current block. The merge index may be signaled by theencoding device. Motion information may include a motion vector and areference picture. When motion information of a temporal neighboringblock is used in the skip mode and the merge mode, a highest picture ina reference picture list may be used as a reference picture.

In the case of the skip mode, a difference (residual) between aprediction sample and an original sample is not transmitted,distinguished from the merge mode.

In the case of the MVP mode, the motion vector of the current block maybe derived using a motion vector of a neighboring block as a motionvector predictor. Here, the neighboring block may include a spatialneighboring block and a temporal neighboring block.

When the merge mode is applied, for example, a merge candidate list canbe generated using a motion vector of a reconstructed spatialneighboring block and/or a motion vector corresponding to a Col blockwhich is a temporal neighboring block. A motion vector of a candidateblock selected from the merge candidate list is used as the motionvector of the current block in the merge mode. The aforementionedinformation on prediction may include a merge index indicating acandidate block having the best motion vector selected from candidateblocks included in the merge candidate list. Here, the predictor (230)may derive the motion vector of the current block using the merge index.

When the MVP (Motion vector Prediction) mode is applied as anotherexample, a motion vector predictor candidate list may be generated usinga motion vector of a reconstructed spatial neighboring block and/or amotion vector corresponding to a Col block which is a temporalneighboring block. That is, the motion vector of the reconstructedspatial neighboring block and/or the motion vector corresponding to theCol block which is the temporal neighboring block may be used as motionvector candidates. The aforementioned information on prediction mayinclude a prediction motion vector index indicating the best motionvector selected from motion vector candidates included in the list.Here, the predictor (230) may select a prediction motion vector of thecurrent block from the motion vector candidates included in the motionvector candidate list using the motion vector index. The predictor ofthe encoding device may obtain a motion vector difference (MVD) betweenthe motion vector of the current block and a motion vector predictor,encode the MVD and output the encoded MVD in the form of a bitstream.That is, the MVD can be obtained by subtracting the motion vectorpredictor from the motion vector of the current block. Here, thepredictor (230) may obtain a motion vector included in the informationon prediction and derive the motion vector of the current block byadding the motion vector difference to the motion vector predictor. Inaddition, the predictor may obtain or derive a reference picture indexindicating a reference picture from the aforementioned information onprediction.

The adder (240) can add a residual sample to a prediction sample toreconstruct a current block or a current picture. The adder (240) mayreconstruct the current picture by adding the residual sample to theprediction sample in units of a block. When the skip mode is applied, aresidual is not transmitted and thus the prediction sample may become areconstructed sample. Although the adder (240) is described as aseparate component, the adder (240) may be a part of the predictor(230). Meanwhile, the adder (240) may be referred to as a reconstructoror reconstructed block generator.

The filter (250) may apply deblocking filtering, sample adaptive offsetand/or ALF to the reconstructed picture. Here, sample adaptive offsetmay be applied in units of a sample after deblocking filtering. The ALFmay be applied after deblocking filtering and/or application of sampleadaptive offset.

The memory (260) may store a reconstructed picture (decoded picture) orinformation necessary for decoding. Here, the reconstructed picture maybe the reconstructed picture filtered by the filter (250). For example,the memory (260) may store pictures used for inter-prediction. Here, thepictures used for inter-prediction may be designated according to areference picture set or a reference picture list. A reconstructedpicture may be used as a reference picture for other pictures. Thememory (260) may output reconstructed pictures in an output order.

Meanwhile, in the case of inter prediction, an inter prediction methodconsidering distortion of an image has been proposed. Specifically, anaffine motion model has been proposed to efficiently derive a motionvector for sub-blocks or sample points of a current block and toincrease accuracy of inter prediction despite deformation of imagerotation, zoom-in or zoom-out. That is, an affine motion model whichderives a motion vector for sub-blocks or sample points of a currentblock has been proposed. Prediction using the affine motion model may becalled affine inter prediction or affine motion prediction.

For example, the affine inter prediction using the affine motion modelmay efficiently express four motions, that is, four deformations, asdescribed below.

FIG. 3 illustrates a motion expressed through the affine motion model.Referring to FIG. 3, a motion that may be represented through the affinemotion model may include a translational motion, a scale motion, arotational motion, and a shear motion. That is, a scale motion in which(a portion of) an image is scaled according to the passage of time, arotational motion in which (a portion of) an image is rotated accordingto the passage of time, and a shear motion in which (a portion of) animage is parallelogrammically deformed according to the passage of time,as well as the translational motion in which (a portion of) an image isplanarly moved according to the passage of time illustrated in FIG. 3,may be effectively represented as illustrated in FIG. 3.

The encoding apparatus/decoding apparatus may predict a distortion shapeof the image based on the motion vectors at control points (CPs) of thecurrent block through the affine inter prediction the compressionperformance of the image may be improved by increasing accuracy ofprediction. In addition, since a motion vector for at least one controlpoint of the current block may be derived using a motion vector of aneighboring block of the current block, a burden of a data amount onadditional information may be reduced and inter prediction efficiencymay be improved considerably.

As an example of the affine inter prediction, motion information atthree control points, that is, three reference points, may be required.

FIG. 4 illustrates the affine motion model in which motion vectors forthree control points are used.

When a top-left sample position in a current block (400) is (0, 0),sample positions (0, 0), (w, 0), and (0, h) may be defined as thecontrol points as shown in FIG. 4. Hereinafter, the control point of thesample position (0, 0) may be represented as CP0, the control point ofthe sample position (w, 0) may be represented as CP1, and the controlpoint of the sample position (0, h) may be represented as CP2.

An equation for the affine motion model may be derived using the controlpoints and the motion vectors of the corresponding control pointsdescribed above. An equation for the affine motion model may beexpressed as follows.

$\begin{matrix}\{ \begin{matrix}{v_{x} = {{\frac{( {v_{1x} - v_{0x}} )}{w}*x} + {\frac{( {v_{2x} - v_{0x}} )}{h}*y} + v_{0x}}} \\{v_{y} = {{\frac{( {v_{1y} - v_{0y}} )}{w}*x} - {\frac{( {v_{2y} - v_{0y}} )}{h}*y} + v_{0y}}}\end{matrix}  & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

Here, w denotes a width of the current block (400), h denotes a heightof the current block (400), v_(0x) and v_(0y) denote an x component andy component of the motion vector of CP0, respectively, v_(1x) and v_(1y)denote an x component and a y component of the motion vector of CP1,respectively, and v_(2x) and v_(2y) denote an x component and a ycomponent of the motion vector of CP2, respectively. In addition, xdenotes an x component of a position of a target sample in the currentblock (400), y denotes a y component of the position of the targetsample in the current block (400), v_(x) denotes an x component of amotion vector of the target sample in the current block (400), and v_(y)denotes a y component of the motion vector of the target sample in thecurrent block (400).

Since the motion vector of CP0, the motion vector of CP1, and the motionvector of CP2 are known, a motion vector based on the sample position inthe current block may be derived based on Equation 1. That is, accordingto the affine motion model, the motion vectors v0(v_(0x), v_(0y)),v1(v_(1x), v_(1y)), and v2(v_(2x), v_(2y)) at the control points may bescaled based on a distance ratio between the coordinates (x, y) of thetarget sample and the three control points to derive the motion vectorsof the target sample according to the position of the target sample.That is, according to the affine motion model, a motion vector of eachsample in the current block may be derived based on the motion vectorsof the control points. Meanwhile, a set of motion vectors of samples inthe current block derived according to the affine motion model may bereferred to as an affine motion vector field (MVF).

Meanwhile, six parameters for Equation 1 may be represented by a, b, c,d, e, and f as shown in Equation 1 below, and an equation for the affinemotion model represented by the six parameters may be as follows.

$\begin{matrix}{a = {{\frac{( {v_{1x} - v_{0x}} )}{w}\mspace{14mu} b} = {{\frac{( {\nu_{2x} - \nu_{0x}} )}{h}\mspace{14mu} c} = v_{0x}}}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack \\{d = {{\frac{( {v_{1y} - v_{0y}} )}{w}\mspace{14mu} e} = {{{- \frac{( {v_{2y} - v_{0y}} )}{h}}\mspace{14mu} f} = \nu_{0y}}}} & \; \\\{ \begin{matrix}{v_{x} = {{a*x} + {b*y} + c}} \\{v_{y} = {{d*x} + {e*y} + f}}\end{matrix}  & \;\end{matrix}$

Here, w denotes a width of the current block (400), h denotes a heightof the current block (400), v_(0x) and v_(0y) denote the x component ofthe motion vector of CP0, y components, v1x and v1y represent an xcomponent and a y component of the motion vector of CP1, respectively,and v_(2x) and v_(2y) represent the x component and the y component ofthe motion vector of CP2, respectively. In addition, x denotes the xcomponent of the position of the target sample in the current block(400), y denotes the y component of the position of the target sample inthe current block (400), v_(x) denotes the x component of the motionvector of the target sample in the current block (400), v_(y) denotesthe y component of the motion vector of the target sample in the currentblock (400).

The affine motion model or the affine inter prediction using the sixparameters may be referred to as a 6-parameter affine motion model orAF6.

In addition, as an example of the affine inter prediction, motioninformation at two control points, i.e., two reference points, may berequired.

FIG. 5 illustrates the affine motion model in which motion vectors fortwo control points are used. The affine motion model using two controlpoints may represent three motions including a translational motion, ascale motion, and a rotational motion. The affine motion modelrepresenting the three motions may be referred to as a similarity affinemotion model or a simplified affine motion model.

When a top-left sample position in a current block (500) is (0, 0),sample positions (0, 0) and (w, 0) may be defined as the control pointsas shown in FIG. 5. Hereinafter, the control point of the sampleposition (0, 0) may be represented as CP0 and the control point of thesample position (w, 0) may be represented as CP1.

An equation for the affine motion model may be derived using the controlpoints and the motion vectors of the corresponding control pointsdescribed above. An equation for the affine motion model may beexpressed as follows.

$\begin{matrix}\{ \begin{matrix}{\nu_{x} = {{\frac{( {v_{1x} - v_{0x}} )}{w}*x} - {\frac{( {v_{1y} - \nu_{0y}} )}{w}*y} + \nu_{0x}}} \\{v_{y} = {{\frac{( {v_{1y} - v_{0y}} )}{w}*x} - {\frac{( {v_{1x} - v_{0x}} )}{w}*y} + v_{0y}}}\end{matrix}  & \lbrack {{Equation}\mspace{14mu} 3} \rbrack\end{matrix}$

Here, w denotes a width of the current block (500), v_(0x) and v_(0y)denote x and y components of the motion vector of CP0, respectively, andv_(1x) and v_(1y) denote x and y components of the motion vector of CP1.In addition, x denotes an x component of a position of a target samplein the current block (500), y denotes a y component of the position ofthe target sample in the current block (500), v_(x) denotes an xcomponent of the motion vector of the target sample in the current block(500), and v_(y) denotes a y component of the motion vector of thetarget sample in the current block (500).

Meanwhile, four parameters of Equation 3 may be represented by a, b, c,and d as in the following Equation, and an equation for the affinemotion model represented by the four parameters may be as follows.

$\begin{matrix}{a = {{\frac{( {\nu_{1x} - v_{0\; x}} )}{w}\mspace{14mu} b} = {{\frac{( {\nu_{1y} - v_{0y}} )}{w}\mspace{14mu} c} = {{v_{0x}\mspace{14mu} d} = v_{0y}}}}} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack \\\{ \begin{matrix}{v_{x} = {{a*x} - {b*y} + c}} \\{v_{y} = {{b*x} + {a*y} + d}}\end{matrix}  & \;\end{matrix}$

Here, w denotes a width of the current block (500), v_(0x) and v_(0y)denote x and y components of the motion vector of CP0, respectively, andv_(1x) and v_(1y) denote x and y components of the motion vector of CP1,respectively. In addition, x denotes an x component of a position of atarget sample in the current block (500), y denotes a y component of theposition of the target sample in the current block (500), v_(x) denotesan x component of the motion vector of the target sample in the currentblock (500) and v_(y) denotes a y component of the motion vector of thetarget sample in the current block (500). The affine motion model usingthe two control points may be represented by four parameters a, b, c,and d as shown in Equation 4, and thus, the affine motion model usingthe four parameters or the affine inter prediction may be referred to asa 4-parameter affine motion model or AF4. That is, according to theaffine motion model, a motion vector of each sample in the current blockmay be derived based on the motion vectors of the control points.Meanwhile, a set of motion vectors of the samples in the current blockderived according to the affine motion model may be referred to as anaffine motion vector field (MVF).

Meanwhile, as described above, a motion vector of a sample unit may bederived through the affine motion model, and thus accuracy of interprediction may be significantly improved. In this case, however,complexity in the motion compensation process may be significantlyincreased.

Accordingly, it may be limited such that a motion vector of a sub-blockunit of the current block, instead of deriving a motion vector of thesample unit, is derived.

FIG. 6 illustrates a method of deriving a motion vector on a sub-blockbasis based on the affine motion model. FIG. 6 illustrates a case wherea size of the current block is 16×16 and a motion vector is derived inunits of 4×4 sub-blocks. The sub-block may be set to various sizes. Forexample, when the sub-block is set to n×n size (n is a positive integer,e.g., n is 4), a motion vector may be derived in units of n×n sub-blocksin the current block based on the affine motion model and variousmethods for deriving a motion vector representing each sub-block may beapplied.

For example, referring to FIG. 6, a motion vector of each sub-block maybe derived using the center or bottom right side sample position of eachsub-block as a representative coordinate. Here, the center bottom rightposition may indicate a sample position positioned on the bottom rightside among four samples positioned at the center of the sub-block. Forexample, when n is an odd number, one sample may be positioned at thecenter of the sub-block, and in this case, the center sample positionmay be used for deriving the motion vector of the sub-block. However,when n is an even number, four samples may be positioned to be adjacentat the center of the sub-block, and in this case, the bottom rightsample position may be used to derive a motion vector. For example,referring to FIG. 6, representative coordinates of each sub-block may bederived as (2, 2), (6, 2), (10, 2), . . . , (14, 14), and encodingapparatus/decoding apparatus may derive the motion vector of eachsub-block by substituting each of the representative coordinates of thesub-blocks into Equation 1 or 3 described above. The motion vectors ofthe sub-blocks in the current block derived through the affine motionmodel may be referred to as affine MVF.

Meanwhile, as an example, the size of the sub-block in the current blockmay be derived based on the following equation.

$\begin{matrix}\{ \begin{matrix}{M = {{clip}\; 3\ ( {4,w,\frac{w*{MvPre}}{\max( {{{abs}( {v_{1\; x} - v_{0\; x}} )},{{abs}( {v_{1\; y} - v_{0\; y}} )}} )}} )}} \\{N = {{clip}\; 3( {4,h,\frac{h*{MvPre}}{\max( {{{abs}( {v_{2x} - v_{0x}} )},{{abs}( {v_{2y} - v_{0y}} )}} )}} )}}\end{matrix}  & \lbrack {{Equation}\mspace{14mu} 5} \rbrack\end{matrix}$

Here, M denotes a width of the sub-block, and N denotes a height of thesub-block. In addition, v_(0x) and v_(0y) denote an x component and a ycomponent of CPMV0 of the current block, v_(1x) and v_(1y) denote an xcomponent and a y component of CPMV1 of the current block, w denotes awidth of the current block, h denotes a height of the current block, andMvPre denotes a motion vector fraction accuracy. For example, the motionvector fraction accuracy may be set to 1/16.

Meanwhile, in the inter prediction using the above-described affinemotion model, that is, the affine motion prediction, may have an affinemerge mode AF_MERGE and an affine inter mode AF_INTER. Here, the affineinter mode may be referred to as an affine MVP mode AF_MVP.

The affine merge mode is similar to an existing merge mode in that MVDfor the motion vector of the control points is not transmitted. That is,similarly to the existing skip/merge mode, the affine merge mode mayrefer to an encoding/decoding method of performing prediction byderiving a CPMV for each of two or three control points from aneighboring block of the current block.

For example, when the AF_MRG mode is applied to the current block, MVs(i.e., CPMV0 and CPMV1) for CP0 and CP1 may be derived from theneighboring block to which the affine mode is applied among theneighboring blocks of the current block. That is, CPMV0 and CPMV1 of theneighboring block to which the affine mode is applied may be derived asmerge candidates, and the merge candidates may be derived as CPMV0 andCPMV1 for the current block.

The affine inter mode may represent inter prediction of performingprediction based on an affine motion vector predictor (MVP) by derivingan MVP for a motion vector of the control points, driving a motionvector of the control points based on a motion vector difference (MOD)and the MVP, and driving an affine MVF of the current block based on themotion vector of the control points. Here, the motion vector of thecontrol point may be represented as a control point motion vector(CPMV), the MVP of the control point may be represented as a controlpoint motion vector predictor (CPMVP), and the MVD of the control pointmay be represented as a control point motion vector difference (CPMVD).Specifically, for example, the encoding apparatus may derive a controlpoint motion vector predictor (CPMVP) and a control point motion vector(CPMV) for each of CP0 and CP1 (or CP0, CP1, and CP2) and transmit orstore information on the CPMVP and/or the CPMVD, which is a differencebetween CPMVP and CPMV.

Here, when the affine inter mode is applied to the current block, theencoding apparatus/decoding apparatus may construct an affine MVPcandidate list based on a neighboring block of the current block, theaffine MVP candidate may be referred to as a CPMVP pair candidate, andthe affine MVP candidate list may be referred to as a CPMVP candidatelist.

In addition, each of the affine MVP candidates may refer to acombination of CPMVPs of CP0 and CP1 in a 4-parameter affine motionmodel and may refer to a combination of CPMVPs of CP0, CP1, and CP2 in a6-parameter affine motion model.

FIG. 7 is a flowchart illustrating an affine motion prediction methodaccording to an embodiment of the present disclosure.

Referring to FIG. 7, the affine motion prediction method may berepresented as follows. When the affine motion prediction method starts,first, a CPMV pair may be obtained (S700). Here, the CPMV pair mayinclude CPMV0 and CPMV1 when using the 4-parameter affine model.

Thereafter, affine motion compensation may be performed based on theCPMV pair (S710), and affine motion prediction may be terminated.

In addition, there may be two affine prediction modes to determine theCPMV0 and the CPMV1. Here, the two affine prediction modes may includean affine inter mode and an affine merge mode. In the affine inter mode,the CPMV0 and the CPMV1 may be clearly determined by signaling twomotion vector difference (MVD) information for the CPMV0 and the CPMV1.Meanwhile, in the affine merge mode, a CPMV pair may be derived withoutMVD information signaling.

In other words, in the affine merge mode, the CPMV of the current blockmay be derived using the CPMV of the neighboring block coded in theaffine mode, and in the case of determining the motion vector in unitsof sub-blocks, the affine merge mode may be referred to as a sub-blockmerge mode.

In the affine merge mode, the encoding apparatus may signal, to thedecoding apparatus, an index of a neighboring block coded in the affinemode for deriving the CPMV of the current block and may further signal adifference value between the CPMV of the neighboring block and the CPMVof the current block. Here, in the affine merge mode, an affine mergecandidate list may be constructed based on a neighboring block, and anindex of the neighboring block may represent a neighboring block to bereferred to in order to derive the CPMV of the current block on theaffine merge candidate list. The affine merge candidate list may bereferred to as a sub-block merge candidate list.

The affine inter mode may be referred to as an affine MVP mode. In theaffine MVP mode, the CPMV of the current block may be derived based on acontrol point motion vector predictor (CPMVP) and a control point motionvector difference (CPMVD). In other words, the encoding apparatus maydetermine the CPMVP for the CPMV of the current block, derive a CPMVDwhich is a difference between the CPMV of the current block and theCPMVP, and signal information on the CPMVP and information on the CPMVDto the decoding apparatus. Here, the affine MVP mode may construct anaffine MVP candidate list based on the neighboring block, and theinformation on the CPMVP may represent a neighboring block to bereferred to in order to derive the CPMVP for the CPMV of the currentblock on the affine MVP candidate list. The affine MVP candidate listmay be referred to as a control point motion vector predictor candidatelist.

For example, in case an affine inter mode of a 6-parameter affine motionmodel is applied, the current block may be encoded as described below.

FIG. 8 is a diagram for describing a method for deriving a motion vectorpredictor at a control point according to an embodiment of the presentdisclosure.

Referring to FIG. 8, a motion vector of CP0 of the current block may beexpressed as v₀, a motion vector of CP1 may be expressed as v₁, a motionvector of a control point of a bottom-left sample position may beexpressed as v₂, and a motion vector of CP2 may be expressed as v₃. Morespecifically, v₀ may denote a CPMVP of CP0, v₁ may denote a CPMVP ofCP1, and v₂ may denote a CPMVP of CP2.

An affine MVP candidate may be a combination of a CPMVP candidate ofCP0, a CPMVP candidate of CP1, and a CPMVP candidate of CP2.

For example, the affine MVP candidate may be derived as described below.

More specifically, a maximum of 12 CPMVP candidate combinations may bedetermined according to the equation shown below.{(v ₀ ,v ₁ ,v ₂)|v ₀ ={v _(A) ,v _(B) ,v _(C) },v ₁ ={v _(D) ,v _(E) },v₂ ={v _(F) ,v _(G)}}  [Equation 6]

Herein, v_(A) may denote a motion vector of neighboring block A, v_(B)may denote a motion vector of neighboring block B, v_(C) may denote amotion vector of neighboring block C, v_(D) may denote a motion vectorof neighboring block D, v_(E) may denote a motion vector of neighboringblock E, v_(F) may denote a motion vector of neighboring block F, andv_(G) may denote a motion vector of neighboring block G.

Additionally, the neighboring block A may represent a neighboring blockpositioned at the top-left of a top-left sample position of the currentblock, the neighboring block B may represent a neighboring blockpositioned at the top of the top-left sample position of the currentblock, and the neighboring block C may represent a neighboring blockpositioned at a left-side of the top-left sample position of the currentblock. Additionally, the neighboring block D may represent a neighboringblock positioned at the top of a top-right sample position of thecurrent block, and the neighboring block E may represent a neighboringblock positioned at the top-right of the top-right sample position ofthe current block. And, the neighboring block F may represent aneighboring block positioned at a left-side of a bottom-left sampleposition of the current block, and the neighboring block G may representa neighboring block positioned at the bottom-left of the bottom-leftsample position of the current block.

More specifically, referring to the above-described Equation 6, theCPMVP candidate of CP0 may include motion vector v_(A) of theneighboring block A, motion vector v_(B) of the neighboring block B,and/or motion vector v_(C) of the neighboring block C, the CPMVPcandidate of CP1 may include motion vector v_(D) of the neighboringblock D, and/or motion vector v_(E) of the neighboring block E, and theCPMVP candidate of CP2 may include motion vector v_(F) of theneighboring block F, and/or motion vector v_(G) of the neighboring blockG.

In other words, CPMVP v₀ of the CP0 may be derived based on a motionvector of at least one of neighboring blocks A, B, and C of the top-leftsample position. Herein, neighboring block A may represent a block beingpositioned at a top-left of a top-left sample position of the currentblock, neighboring block B may represent a block being positioned at atop of the top-left sample position of the current block, andneighboring block C may represent a block being positioned at aleft-side of the top-left sample position of the current block.

A combination of a maximum of 12 CPMVP candidates including CPMVPcandidates of the CP0, CPMVP candidates of the CP1, and CPMVP candidatesof the CP2 may be derived based on the motion vectors of the neighboringblocks.

Thereafter, the derived combination of CPMVP candidates may be alignedby order of candidates having lower DV values. Thus, the top 2 CPMVPcandidate combinations may be derived as the affine MVP candidates.

The DV of a CPMVP candidate combination may be derived by using thefollowing equation.DV=|(v _(1x) −v _(0x))*h−(v2_(y) −v0_(y))*w|+|(v1_(y) −v0_(y))*h+(v2_(x)−v0_(x))*w|  [Equation 7]

Thereafter, the encoding apparatus may determine CPMVs for each of theaffine MVP candidates. Then, by comparing Rate Distortion (RD) costs forthe CPMVs, affine MVP candidates having the lower RD costs may beselected as the optimal affine MVP candidates for the current block. Theencoding apparatus may encode and signal indexes indicating the optimalcandidates and CPMVDs.

Additionally, for example, in case the affine merge mode is applied, thecurrent block may be encoded as described below.

FIG. 9 is a diagram for describing a method for deriving a motion vectorpredictor at a control point according to an embodiment of the presentdisclosure.

An affine merge candidate list of the current block may be constructedbased on neighboring blocks of the current block shown in FIG. 9. Theneighboring blocks may include neighboring block A, neighboring block B,neighboring block C, neighboring block D, and neighboring block E. And,the neighboring block A may denote a left neighboring block of thecurrent block, the neighboring block B may denote a top neighboringblock of the current block, the neighboring block C may denote atop-right corner neighboring block of the current block, the neighboringblock D may denote a bottom-left corner neighboring block of the currentblock, and the neighboring block E may denote a top-left cornerneighboring block of the current block.

For example, in case the size of the current block is W×H, and, in case,the x component of the top-left sample position of the current block is0 and the y component is 0, the left neighboring block may be a blockincluding a sample of coordinates (−1, H−1), the top neighboring blockmay be a block including a sample of coordinates (W−1, −1), thetop-right corner neighboring block may be a block including a sample ofcoordinates (W, −1), the bottom-left corner neighboring block may be ablock including a sample of coordinates (−1, H), and the top-left cornerneighboring block may be a block including a sample of coordinates (−1,−1).

More specifically, for example, an encoding apparatus may scanneighboring block A, neighboring block B, neighboring block C,neighboring block D, and neighboring block E of the current block by aspecific scanning order, and, in the scanning order, the neighboringblock being the first to be encoded to the affine prediction mode may bedetermined as a candidate block of the affine merge mode, i.e., anaffine merge candidate. Herein, for example, the specific scanning orderay be an alphabetical order. More specifically, the specific scanningorder may be neighboring block A, neighboring block B, neighboring blockC, neighboring block D, and neighboring block E.

Thereafter, the encoding apparatus may determine an affine motion modelof the current block by using the determined candidate block, determinea CPMV of the current block based on the affine motion model, anddetermine an affine MVF of the current block based on the CPMV.

For example, in case neighboring block A is determined as the candidateblock of the current block, neighboring block A may be coded asdescribed below.

FIG. 10 illustrates an example of affine prediction being performed in acase where neighboring block A is selected as an affine merge candidate.

Referring to FIG. 10, an encoding apparatus may determine neighboringblock A of the current block as a candidate block, and the encodingapparatus may derive an affine motion model of the current block basedon CPMV, v₂, and v₃ of the neighboring block. Thereafter, the encodingapparatus may determine CPMV, v₀, and v₁ of the current block based onthe affine motion model. The encoding apparatus may determine an affineMVF based on the CPMV, v₀, and v₁ of the current block and may performan encoding process for the current block based on the affine MVF.

Meanwhile, in relation with affine inter prediction, inherited affinedcandidates are being considered for the construction of the affine MVPcandidate list.

Herein, the inherited affine candidates may be as described below.

For example, in case a neighboring block of the current block is anaffine block, and in case a reference picture of the current block and areference picture of the neighboring block are the same, an affine MVPpair of the current block may be determined from the affine motion modelof the neighboring block. Herein, the affine block may represent a blockto which the affine inter prediction is applied. The inherited affinecandidates may denote CPMVPs (e.g., the affine MVP pair) derived basedon the affine motion model of the neighboring block.

More specifically, for example, the inherited affined candidates may bederived as described below.

FIG. 11 illustrates exemplary neighboring blocks for deriving theinherited affine candidates.

Referring to FIG. 11, the neighboring blocks of the current block mayinclude a left neighboring block A0 of the current block, a bottom-leftcorner neighboring block A1 of the current block, a top neighboringblock B0 of the current block, a top-right corner neighboring block B1of the current block, and a top-left corner neighboring block B2 of thecurrent block.

For example, in case the size of the current block is W×H, and, in case,the x component of the top-left sample position of the current block is0 and the y component is 0, the left neighboring block may be a blockincluding a sample of coordinates (−1, H−1), the top neighboring blockmay be a block including a sample of coordinates (W−1, −1), thetop-right corner neighboring block may be a block including a sample ofcoordinates (W, −1), the bottom-left corner neighboring block may be ablock including a sample of coordinates (−1, H), and the top-left cornerneighboring block may be a block including a sample of coordinates (−1,−1).

An encoding apparatus/decoding apparatus may sequentially checkneighboring blocks A0, A1, B0, B1, and B2. And, in case a neighboringblock is coded by using an affine motion model, and, in case a referencepicture of the current block and a reference picture of the neighboringblock are the same, 2 CPMVs or 3 CPMVs of the current block may bederived based on the affine motion model of the neighboring block. TheCPMVs may be derived as affine MVP candidates of the current block. Theaffine MVP candidates may represent the inherited affine candidates.

For example, a maximum of two inherited affine candidates may be derivedbased on the neighboring blocks.

For example, the encoding apparatus/decoding apparatus may derive afirst affine MVP candidate based on a first block within the neighboringblocks. Herein, the first block may be coded by using an affine motionmodel, and a reference picture of the first block and a referencepicture of the current block may be the same. More specifically, whenchecking the neighboring blocks by a specific order, the first block maybe a first block that is verified to be satisfying the conditions. Theconditions may be that coding is performed by using the affine motionmodel, and that a reference picture of the block is the same as thereference picture of the current block.

Thereafter, the encoding apparatus/decoding apparatus may derive asecond affine MVP candidate based on a second block within theneighboring blocks. Herein, the second block may be coded by using anaffine motion model, and a reference picture of the second block and areference picture of the current block may be the same. Morespecifically, when checking the neighboring blocks by a specific order,the second block may be a second block that is verified to be satisfyingthe conditions. The conditions may be that coding is performed by usingthe affine motion model, and that a reference picture of the block isthe same as the reference picture of the current block.

Alternatively, for example, a maximum of one inherited affine candidatemay be derived based on the neighboring blocks.

For example, the encoding apparatus/decoding apparatus may derive afirst affine MVP candidate based on a first block within the neighboringblocks. Herein, the first block may be coded by using an affine motionmodel, and a reference picture of the first block and a referencepicture of the current block may be the same. More specifically, whenchecking the neighboring blocks by a specific order, the first block maybe a first block that is verified to be satisfying the conditions. Theconditions may be that coding is performed by using the affine motionmodel, and that a reference picture of the block is the same as thereference picture of the current block.

A source code for an MVP candidate derivation of the current block maybe derived as shown below in the following table.

TABLE 1 addAffineMVPCandUnscaled(pu, eRefPicList, refIdx, posLB,MD_LEFT, affiAMVPInfo); if (affiAMVPInfo.numCand < 2)addAffineMVPCandUnscaled(pu, eRefPicList, refIdx, posRT, MD_ABOVE,affiAMVPInfo); if (affiAMVPInfo.numCand < 2)addAffineMVPCandUnscaled(pu, eRefPicList, refIdx, posRT, MD_ABOVE_RIGHT,affiAMVPInfo); if (affiAMVPInfo.numCand < 2)addAffineMVPCandUnscaled(pu, eRefPicList, refIdx, posLB, MD_BELOW_LEFT,affiAMVPInfo); if (affiAMVPInfo.numCand < 2)addAffineMVPCandUnscaled(pu, eRefPicList, refIdx, posLT, MD_ABOVE_LEFT,affiAMVPInfo);

Referring to Table 1, the encoding apparatus/decoding apparatus maydetermine whether or not the left neighboring block is coded by usingthe affine motion model, and may also determine whether or not areference picture of the current block and a reference picture of theleft neighboring block are the same. In case the above-describedconditions are satisfied, CPMVs that are derived based on the affinemotion model of the left neighboring block may be derived as CPMVPcandidates of the current block.

Thereafter, the encoding apparatus/decoding apparatus may determinewhether a number of the derived CPMVP candidates is less than 2. In casethe number of derived CPMVP candidates is not less than 2, the CPMVPcandidate derivation process may be ended.

Additionally, in case the number of derived CPMVP candidates is lessthan 2, it may be determined whether or not the top neighboring block iscoded by using the affine motion model and whether or not a referencepicture of the current block and a reference picture of the topneighboring block are the same. And, in case the above-describedconditions are satisfied, CPMVs that are derived based on the affinemotion model of the top neighboring block may be derived as CPMVPcandidates of the current block.

Thereafter, the encoding apparatus/decoding apparatus may determinewhether a number of the derived CPMVP candidates is less than 2. In casethe number of derived CPMVP candidates is not less than 2, the CPMVPcandidate derivation process may be ended.

Additionally, in case the number of derived CPMVP candidates is lessthan 2, it may be determined whether or not the top-right cornerneighboring block is coded by using the affine motion model and whetheror not a reference picture of the current block and a reference pictureof the top-right corner neighboring block are the same. And, in case theabove-described conditions are satisfied, CPMVs that are derived basedon the affine motion model of the top-right corner neighboring block maybe derived as CPMVP candidates of the current block.

Thereafter, the encoding apparatus/decoding apparatus may determinewhether a number of the derived CPMVP candidates is less than 2. In casethe number of derived CPMVP candidates is not less than 2, the CPMVPcandidate derivation process may be ended.

Additionally, in case the number of derived CPMVP candidates is lessthan 2, it may be determined whether or not the bottom-left cornerneighboring block is coded by using the affine motion model and whetheror not a reference picture of the current block and a reference pictureof the bottom-left corner neighboring block are the same. And, in casethe above-described conditions are satisfied, CPMVs that are derivedbased on the affine motion model of the bottom-left corner neighboringblock may be derived as CPMVP candidates of the current block.

Thereafter, the encoding apparatus/decoding apparatus may determinewhether a number of the derived CPMVP candidates is less than 2. In casethe number of derived CPMVP candidates is not less than 2, the CPMVPcandidate derivation process may be ended.

Additionally, in case the number of derived CPMVP candidates is lessthan 2, it may be determined whether or not the top-left cornerneighboring block is coded by using the affine motion model and whetheror not a reference picture of the current block and a reference pictureof the top-left corner neighboring block are the same. And, in case theabove-described conditions are satisfied, CPMVs that are derived basedon the affine motion model of the top-left corner neighboring block maybe derived as CPMVP candidates of the current block.

Additionally, as another example, the inherited affine candidate may bederived as described below.

For example, the encoding apparatus/decoding apparatus may sequentiallycheck the neighboring blocks by a specific order. And, in case aneighboring block is coded by using an affine motion model, and, in casea reference picture of the current block and a reference picture of theneighboring block are the same, an inherited affine candidate that doesnot apply scaling based on the neighboring block may be derived. And, incase the neighboring block is coded by using an affine motion model,and, in case a reference picture of the current block and a referencepicture of the neighboring block are not the same, an inherited affinecandidate applying scaling based on the neighboring block may bederived.

More specifically, in case the neighboring block is coded by using anaffine motion model, and, in case a reference picture of the currentblock and a reference picture of the neighboring block are the same, asdescribed above in the previous embodiment, the encodingapparatus/decoding apparatus may derive the affine MVP candidate of thecurrent block based on the affine motion model of the neighboring block.

Additionally, in case the neighboring block is coded by using an affinemotion model, and, in case a reference picture of the current block anda reference picture of the neighboring block are not the same, theencoding apparatus/decoding apparatus may derive motion vectors for CPsof the current block based on the affine motion model of the neighboringblock, may scale the motion vectors by using a scaling factor, and mayderive the scaled motion vectors as the affine MVP candidates. Herein,the scaling factor may be a distance ratio of a first time distance anda second time distance. More specifically, the scaling factor may be avalue obtained by dividing the first time distance by the second timedistance. The first time distance may be a difference between a PictureOrder Count (POC) of a current picture including the current block and aPOC of a reference picture of the current block. And, the second timedistance may be a difference between a POC of the current picture and aPOC of a reference picture of the neighboring block.

A source code for the above-described MVP candidate derivation of thecurrent block may be derived as shown below in the following table.

TABLE 2  Bool isScaledFlagIX = false;  const PredictionUnit* tmpPU =cs.getPURestricted( posl B.offset( −1, 1 ), pu, pu.chType ); isScaledFlagIX = tmpPU != NULL && CU::isInter( *tmpPU->cu ) &&tmpPU->cu->affine;  if ( !isScaledFlagLX )  {   tmpPU =cs.getPURestricted( posLB.offset( −1, 0 ), pu, pu.chType );  isScaledFlagLX = tmpPU != NULL && CU::isInter( *tmpPU->cu ) &&tmpPU->cu->affine;  }  // Left predictor search  if ( isScaledFlagLX ) {   Bool bAdded = addAffineMVPCandUnscaled( pu, eRefPicList, refIdx,posLB, MD_BELOW_LEFT, affiAMVPInfo );   if ( !bAdded )   {    bAdded =addAffineMVPCandUnscaled( pu, eRefPicList, refIdx, posLB, MD_LEFT,affiAMVPInfo );    if ( !bAdded )    {     bAdded =addAffineMVPCandWithScaling( pu, eRefPicList, refIdx, posLB,MD_BELOW_LEFT, affiAMVPInfo );     if ( !bAdded )     {     addAffineMVPCandWithScaling( pu, eRefPicList, refIdx, posLB,MD_LEFT, affiAMVPInfo );     }    }   }  }  // Above predictor search  {  Bool bAdded = addAffineMVPCandUnscaled( pu, eRefPicList, refIdx,posRT, MD_ABOVE_RIGHT, affiAMVPInfo );   if ( !bAdded )   {    bAdded -addAffineMVPCandUnscaled( pu, eRefPicList, refIdx, posRT, MD_ABOVE,affiAMVPInfo );    if ( !bAdded )    {     addAffineMVPCandUnscaled( pu,eRefPicList, refIdx, posLT, MD_ABOVE_LEFT, affiAMVPInfo );    }   }  } if ( !isScaledFlagLX )  {   Bool bAdded = addAffineMVPCandWithScaling(pu, eRefPicList, refIdx, posRT, MD_ABOVE_RIGHT, affiAMVPInfo );   if (!bAdded )   {    bAdded = addAffineMVPCandWithScaling( pu, eRefPicList,refIdx, posRT, MD_ABOVE, affiAMVPInfo );    if ( !bAdded )    {    addAffineMVPCandWithScaling( pu, eRefPicList, refIdx, posLT,MD_ABOVE_LEFT, affiAMVPInfo );    }   }  }

The performance and encoding/decoding time of the above-describedembodiments are very similar. And, therefore, after performing acomplexity analysis, the embodiment having the lower complexity levelmay be selected.

The complexity analysis for the embodiments may be derived as shownbelow in the following table.

TABLE 3 Number of operations for 4-parameter candidates pruning 4 comp.Number of operations for 6-parameter candidates pruning 6 comp. Numberof operations in inheriting 4-parameter neighbor 10 shift + 10 add (2shift + 2 add for deriving parameters, (4 shift + 4 add) * 2 forderiving CPMVPs] Number of operations in inheriting 6-parameter neighbor16 shift + 16 add (4 shift + 4 add for deriving parameters, (4 shift + 4add) * 3 for deriving CPMVPs) Inherited affine AMVP candidates Maxnumber Max number Scaled version of potential of potential of motioncandidate inherited vector (Y/N) positions candidates How to inheritneighbor motion model Summary method 1 N 5 pos. 10 cand.: non-scaled (4-parameter) 5 cand. in list0 and 5 pos. 5 cand. in list1 10 cand. 36comp. 100 shift. 100 add. method 1 N 5 pos. 10 cand.: No distinguish in4/5 parameter model non-scaled (6- parameter) 5 cand. in list0 andAlways use LT, RT, LB contorl points of neighbor 5 pos. 5 cand. in list1affine coded blcok to calculte 2 or 3 CPMVPs of 10 cand. current blockby 6 parameter motion model 54 comp. LB CPMV of 4 parameter affine blockshould be 160 shift. calculated to generate pseudo 6-parameter model 160add. method 2 Y 5 pos. 2 cand.: scaled (4- parameter) left predictor and5 pos. above predictor 2 cand. 4 comp. 20 shfit. 20 add. method 2 Y 5pos. 2 cand.: No distinguish in 4/5 parameter model scaled (6 parameter)left predictor and Always use LT, RT, LB contorl points of neighbor 5pos. above predictor affine coded blcok to calculte 2 or 3 CPMVPs of 2cand. current block by 6 parameter motion model 6 comp. LB CPMV of 4parameter affine block should be 32shfit. calculated to generate pseudo6-parameter model 32 add.

Referring to Table 3, unlike the second embodiment, which considers bothscaled affine MVP candidates and non-scaled affine MVP candidates, thefirst embodiment, which considers only non-scaled affine MVP candidates,may be excellent in the aspect of having smaller numbers comp, shift,and add.

Meanwhile, the present disclosure proposes a method having similarperformance and encoding/decoding times as the above-describedembodiments while having specifically lower complexity.

For example, a left predictor and an above predictor are divided, andthe left predictor may be derived based on an affine coded block havingthe same reference picture as the current block, which is first derivedby scanning a left neighboring block and a bottom-left cornerneighboring block by a specific order, and the above predictor may bederived based on an affine coded block having the same reference pictureas the current block, which is first derived by scanning a top-rightcorner neighboring block, a top neighboring block, and a top-left cornerneighboring block by a specific order.

More specifically, the encoding apparatus/decoding apparatus may derivea first affine MVP candidate of the current block from a left blockgroup including a left neighboring block and a bottom-left cornerneighboring block, and the encoding apparatus/decoding apparatus mayderive a second affine MVP candidate of the current block from a topblock group including a top-right corner neighboring block, a topneighboring block, and a top-left corner neighboring block.

Herein, the first affine MVP candidate may be derived based on a firstblock within the left block group, the first block may be coded by usingan affine motion model, and a reference picture of the first block and areference picture of the current block may be the same. Morespecifically, the first block may be a first block that is verified tobe satisfying the conditions, when checking neighboring blocks withinthe left block group by a specific order. The conditions may be thatcoding is performed by using an affine motion model, and that areference picture of a block is the same as a reference picture of thecurrent block.

Additionally, the second affine MVP candidate may be derived based on asecond block within the top block group, the second block may be codedby using an affine motion model, and a reference picture of the secondblock and a reference picture of the current block may be the same. Morespecifically, the second block may be a first block that is verified tobe satisfying the conditions, when checking neighboring blocks withinthe top block group by a specific order. The conditions may be thatcoding is performed by using an affine motion model, and that areference picture of a block is the same as a reference picture of thecurrent block.

Additionally, for example, an order for scanning the left neighboringblock and the bottom-left corner neighboring block may be an order ofleft neighboring block and bottom-left corner neighboring block. And, anorder for scanning the top-right corner neighboring block, the topneighboring block, and the top-left corner neighboring block may be anorder of top-right corner neighboring block, top neighboring block, andtop-left corner neighboring block. Additionally, other than theabove-described example, another example of an order for scanning theabove-described neighboring block may be used.

A source code for the above-described affine MVP candidate derivation ofthe current block may be derived as shown below in the following table.

TABLE 4 // Left predictor search {  Bool bAdded =addAffineMVPCandUnscaled( pu, eRefPicList,  refIdx, posLB,MD_BELOW_LEFT, affiAMVPInfo );  if ( !bAdded )  {   bAdded =addAffineMVPCandUnscaled( pu, eRefPicList, refIdx,   posLB, MD_LEFT,affiAMVPInfo );  } } // Above predictor search {  Bool bAdded =addAffineMVPCandUnscaled( pu, eRefPicList, refIdx,  posRT,MD_ABOVE_RIGHT, affiAMVPInfo );  if ( !bAdded )  {   bAdded =addAffineMVPCandUnscaled( pu, eRefPicList, refIdx,   posRT, MD_ABOVE,affiAMVPInfo );   if ( !bAdded )   {    addAffineMVPCandUnscaled( pu,eRefPicList, refIdx, posLT,    MD_ABOVE_LEFT, affiAMVPInfo );   }  } }

Additionally, the complexity analysis for the affine MVP candidatederivation method proposed in the present disclosure may be derived asshown below in the following table.

TABLE 5 Number of operations for 4-parameter candidates pruning 4 comp.Number of operations for 6-parameter candidates pruning 6 comp. Numberof operations in inheriting 4-parameter neighbor 10 shift + 10 add (2shift + 2 add for deriving parameters, (4 shift + 4 add) * 2 forderiving CPMVPs) Number of operations in inheriting 6-parameter neighbor16 shift + 16 add (4 shift + 4 add for deriving parameters, (4 shift + 4add) * 3 for deriving CPMVPs) Inherited affine AMVP candidates Maxnumber Max number Scaled version of potential of potential of motioncandidate inherited vector (Y/N) positions candidates How to inheritneighbor motion model Summary Proposed Y 5 pos. 2 cand.: non-scaledMethod left predictor and 5 pos. (4-parameter) above predictor 2 cand. 4comp. 20 shfit. 20 add. Proposed Y 5 pos. 2 cand.: non-scaled Methodleft predictor and 5 pos. (6 parameter) above predictor 2 cand. 6 comp.32 shfit. 32 add.

Referring to Table 5, the proposed affine MVP candidate derivationmethod is performed by considering the smallest numbers of pos, cand,comp, shift, and add as compared to the existing (or old) embodiments,and by considering only the non-scaled affine MVP candidates.Accordingly, in comparison with the above-mentioned embodiments, theproposed method may have the lowest complexity. Therefore, since theaffine MVP candidate derivation method proposed in the presentdisclosure is similar to the existing (or old) embodiments in light ofperformance and encoding/decoding times, while having the lowestcomplexity, it may be determined that the proposed method is moreexcellent than the existing embodiments.

Meanwhile, for example, in case the available number of inheritedaffined candidates is less than 2, a constructed affine candidate may beconsidered. The constructed affine candidate may be derived as describedbelow.

FIG. 12 illustrates exemplary spatial candidates for the constructedaffine candidates

As shown in FIG. 12, motion vectors of neighboring blocks of the currentblock may be divided into 3 groups. Referring to FIG. 12, theneighboring blocks may include neighboring block A, neighboring block B,neighboring block C, neighboring block D, neighboring block E,neighboring block F, and neighboring block G.

The neighboring block A may denote a neighboring block positioned at atop-left of a top-left sample position of the current block, theneighboring block B may denote a neighboring block positioned at a topof the top-left sample position of the current block, and theneighboring block C may denote a neighboring block positioned at aleft-side of the top-left sample position of the current block.Additionally, the neighboring block D may denote a neighboring blockpositioned at a top of a top-right sample position of the current block,and the neighboring block E may denote a neighboring block positioned ata top-right of the top-right sample position of the current block.Additionally, the neighboring block F may denote a neighboring blockpositioned at a left side of a bottom-left sample position of thecurrent block, and the neighboring block G may denote a neighboringblock positioned at a bottom-left of the bottom-left sample position ofthe current block.

For example, the 3 groups may include S₀, S₁, and S₂, and the S₀, theS₁, and the S₂ may be derived as shown below in the following table.

TABLE 6 S₀ = {mv_(A), mv_(B), mv_(C) } S₁ = {mv_(D), mv_(E)) S₂ ={mv_(F), mv_(G))

Herein, mv_(A) may represent a motion vector of the neighboring block A,mv_(B) may represent a motion vector of the neighboring block B, mv_(C)may represent a motion vector of the neighboring block C, mv_(D) mayrepresent a motion vector of the neighboring block D, mv_(E) mayrepresent a motion vector of the neighboring block E, mv_(E) mayrepresent a motion vector of the neighboring block F, and mv_(G) mayrepresent a motion vector of the neighboring block G. The S₀ may denotea first group, the S₁ may denote a second group, and the S₂ may denote athird group.

The encoding apparatus/decoding apparatus may derive mv₀ from the S₀,mv₁ from the S₁, and mv₂ from the S₂, and the encodingapparatus/decoding apparatus may derive an affine MVP candidateincluding the mv₀, the mv₁, and the mv₂. The affine MVP candidate mayrepresent the constructed affine candidate Additionally, the mv₀ may bea CPMVP candidate of CP0, the mv₁ may be a CPMVP candidate of CP1, andthe mv₂ may be a CPMVP candidate of CP2.

Herein, a reference picture for the mv₀ may be the same as a referencepicture of the current block. More specifically, when checking motionvectors within the S₀ according to a specific order, the mv₀ may be afirst motion vector that is verified to be satisfying a condition. Thecondition may be that a reference picture for a motion vector is thesame as a reference picture of the current block.

Additionally, a reference picture for the mv₁ may be the same as areference picture of the current block. More specifically, when checkingmotion vectors within the S₁ according to a specific order, the mv₁ maybe a first motion vector that is verified to be satisfying a condition.The condition may be that a reference picture for a motion vector is thesame as a reference picture of the current block.

Additionally, a reference picture for the mv₂ may be the same as areference picture of the current block. More specifically, when checkingmotion vectors within the S₂ according to a specific order, the mv₂ maybe a first motion vector that is verified to be satisfying a condition.The condition may be that a reference picture for a motion vector is thesame as a reference picture of the current block.

Meanwhile, in case only the mv₀ and the mv₁ are derived, the mv₂ may bederived by using the following equation.

$\begin{matrix}{{{\overset{\_}{mv}}_{2}^{x} = {{\overset{\_}{mv}}_{0}^{x} - {h\frac{( {{\overset{\_}{mv}}_{1}^{y} - {\overset{\_}{mv}}_{0}^{y}} )}{w}}}},{{\overset{\_}{mv}}_{2}^{y} = {{\overset{\_}{mv}}_{0}^{y} + {h\;\frac{( {{\overset{\_}{mv}}_{1}^{x} - {\overset{\_}{mv}}_{0}^{x}} )}{w}}}}} & \lbrack {{Equation}\mspace{14mu} 8} \rbrack\end{matrix}$

Herein, mv₂ ^(x) denotes an x component of the mv₂, mv₂ ^(y) denotes a ycomponent of the mv₂, mv₀ ^(x) denotes an x component of the mv₀, mv₀^(y) denotes a y component of the mv₀, mv₁ ^(x) denotes an x componentof the mv₁, and mv₁ ^(y) denotes a y component of the mv₁. Additionally,w denotes a width of the current block, and h denotes a height of thecurrent block.

Meanwhile, in case only the mv₀ and the mv₂ are derived, the mv₁ may bederived by using the following equation.

$\begin{matrix}{{{\overset{\_}{mv}}_{1}^{x} = {{\overset{\_}{mv}}_{0}^{x} + {h\frac{( {{\overset{\_}{mv}}_{2}^{y} - {\overset{\_}{mv}}_{0}^{y}} )}{w}}}},{{\overset{\_}{mv}}_{1}^{y} = {{\overset{\_}{mv}}_{0}^{y} - {h\;\frac{( {{\overset{\_}{mv}}_{2}^{x} - {\overset{\_}{mv}}_{0}^{x}} )}{w}}}}} & \lbrack {{Equation}\mspace{14mu} 9} \rbrack\end{matrix}$

Herein, mv₁ ^(x) denotes an x component of the mv₁, mv₁ ^(y) denotes a ycomponent of the mv₁, mv₀ ^(x) denotes an x component of the mv₀, mv₀^(y) denotes a y component of the mv₀, mv₂ ^(x) denotes an x componentof the mv₂, and mv₂ ^(y) denotes a y component of the mv₂. Additionally,w denotes a width of the current block, and h denotes a height of thecurrent block.

Additionally, in case the number of available inherited affinecandidates and/or the number of available constructed affine candidatesis less than 2, an AMVP process of the existing HEVC standard may beapplied to the affine MVP list construction. More specifically, in casethe number of available inherited affine candidates and/or the number ofavailable constructed affine candidates is less than 2, a process ofconfiguring MVP candidates of the existing HEVC standard may beperformed.

Meanwhile, flow charts of the embodiments for constructing theabove-described affine MVP list are as described below.

FIG. 13 illustrates an example for constructing an affine MVP list.

Referring to FIG. 13, an encoding apparatus/decoding apparatus may addan inherited candidate to an affine MVP list of a current block (S1300).The inherited candidate may represent the above-described inheritedaffine candidate.

More specifically, the encoding apparatus/decoding apparatus may derivea maximum of 2 inherited affine candidates from neighboring blocks ofthe current block (S1305). Herein, the neighboring blocks may include aleft neighboring block A0 of the current block, a bottom-left cornerneighboring block A1 of the current block, a top neighboring block B0 ofthe current block, a top-right corner neighboring block B1 of thecurrent block, and a top-left corner neighboring block B2 of the currentblock.

For example, the encoding apparatus/decoding apparatus may derive afirst affine MVP candidate based on a first block within the neighboringblocks. Herein, the first block may be coded by using an affine motionmodel, and a reference picture of the first block and a referencepicture of the current block may be the same. More specifically, whenchecking the neighboring blocks by a specific order, the first block maybe a first block that is verified to be satisfying the conditions. Theconditions may be that coding is performed by using the affine motionmodel, and that a reference picture of the block is the same as thereference picture of the current block.

Thereafter, the encoding apparatus/decoding apparatus may derive asecond affine MVP candidate based on a second block within theneighboring blocks. Herein, the second block may be coded by using anaffine motion model, and a reference picture of the second block and areference picture of the current block may be the same. Morespecifically, when checking the neighboring blocks by a specific order,the second block may be a second block that is verified to be satisfyingthe conditions. The conditions may be that coding is performed by usingthe affine motion model, and that a reference picture of the block isthe same as the reference picture of the current block.

Meanwhile, the specific order may be left neighboring blockA0→bottom-left corner neighboring block A1→top neighboring blockB0→top-right corner neighboring block B1→top-left corner neighboringblock B2. Additionally, the checking may be performed by an order otherthan the above-described order and may not be limited only to theabove-described example.

The encoding apparatus/decoding apparatus may add a constructedcandidate to an affine MVP list of the current block (S1310). Theconstructed candidate may represent the above-described constructedaffine candidate. In case a number of available inherited candidates isless than 2, the encoding apparatus/decoding apparatus may add aconstructed candidate to an affine MVP list of the current block.

The encoding apparatus/decoding apparatus may derive mv₀ from a firstgroup, mv₁ from a second group, and mv₂ from a third group, and theencoding apparatus/decoding apparatus may derive the constructed affinecandidate including the mv₀, the mv₁, and the mv₂. The mv₀ may be aCPMVP candidate of CP0, the mv₁ may be a CPMVP candidate of CP1, and themv₂ may be a CPMVP candidate of CP2.

Herein, a reference picture for the mv₀ may be the same as a referencepicture of the current block. More specifically, when checking motionvectors within the first group according to a specific order, the mv₀may be a first motion vector that is verified to be satisfying acondition. The condition may be that a reference picture for a motionvector is the same as a reference picture of the current block.Additionally, a reference picture for the mv₁ may be the same as areference picture of the current block. More specifically, when checkingmotion vectors within the second group according to a specific order,the mv₁ may be a first motion vector that is verified to be satisfying acondition. The condition may be that a reference picture for a motionvector is the same as a reference picture of the current block.Additionally, a reference picture for the mv₂ may be the same as areference picture of the current block. More specifically, when checkingmotion vectors within the third group according to a specific order, themv₂ may be a first motion vector that is verified to be satisfying acondition. The condition may be that a reference picture for a motionvector is the same as a reference picture of the current block.

Additionally, the first group may include a motion vector of theneighboring block A, a motion vector of the neighboring block B, and amotion vector of the neighboring block C. The second group may include amotion vector of the neighboring block D and a motion vector of theneighboring block E. The third group may include a motion vector of theneighboring block F and a motion vector of the neighboring block G. Theneighboring block A may denote a neighboring block positioned at atop-left of a top-left sample position of the current block, theneighboring block B may denote a neighboring block positioned at a topof the top-left sample position of the current block, and theneighboring block C may denote a neighboring block positioned at aleft-side of the top-left sample position of the current block.Additionally, the neighboring block D may denote a neighboring blockpositioned at a top of a top-right sample position of the current block,and the neighboring block E may denote a neighboring block positioned ata top-right of the top-right sample position of the current block.Additionally, the neighboring block F may denote a neighboring blockpositioned at a left side of a bottom-left sample position of thecurrent block, and the neighboring block G may denote a neighboringblock positioned at a bottom-left of the bottom-left sample position ofthe current block.

The encoding apparatus/decoding apparatus may add an HEVC AMVP candidateto the affine MVP list of the current block (S1320). In case a number ofavailable inherited candidates and/or constructed candidates is lessthan 2, the encoding apparatus/decoding apparatus may add an HEVC AMVPcandidate to the affine MVP list of the current block. Morespecifically, in case the number of available inherited candidatesand/or constructed candidates is less than 2, the encodingapparatus/decoding apparatus may perform a process of configuring MVPcandidates of the existing HEVC standard.

FIG. 14 illustrates an example for constructing an affine MVP list.

Referring to FIG. 14, an encoding apparatus/decoding apparatus may addan inherited candidate to an affine MVP list of a current block (S1400).The inherited candidate may represent the above-described inheritedaffine candidate.

More specifically, the encoding apparatus/decoding apparatus may derivea first affine MVP candidate of the current block from a left blockgroup including a left neighboring block and a bottom-left cornerneighboring block (S1405), and the encoding apparatus/decoding apparatusmay derive a second affine MVP candidate of the current block from a topblock group including a top-right corner neighboring block, a topneighboring block, and a top-left corner neighboring block (S1410).

Herein, the first affine MVP candidate may be derived based on a firstblock within the left block group, the first block may be coded by usingan affine motion model, and a reference picture of the first block and areference picture of the current block may be the same. Morespecifically, when checking the neighboring blocks by a specific order,the first block may be a first block that is verified to be satisfyingthe conditions. The conditions may be that coding is performed by usingthe affine motion model, and that a reference picture of the block isthe same as the reference picture of the current block.

Additionally, the second affine MVP candidate may be derived based on asecond block within the top block group, the second block may be codedby using an affine motion model, and a reference picture of the secondblock and a reference picture of the current block may be the same. Morespecifically, when checking the neighboring blocks by a specific order,the second block may be a second block that is verified to be satisfyingthe conditions. The conditions may be that coding is performed by usingthe affine motion model, and that a reference picture of the block isthe same as the reference picture of the current block.

Meanwhile, the specific order for checking the left block group may bean order of left neighboring block and bottom-left corner neighboringblock. Alternatively, the specific order for checking the left blockgroup may be an order of bottom-left corner neighboring block and leftneighboring block. Additionally, the specific order for checking the topblock group may be an order of top-right corner neighboring block, topneighboring block, and top-left corner neighboring block. Alternatively,the specific order for checking the top block group may be an order oftop neighboring block, top-right corner neighboring block, and top-leftcorner neighboring block. Additionally, the checking may be performed byan order other than the above-described order and may not be limitedonly to the above-described example.

The encoding apparatus/decoding apparatus may add a constructedcandidate to an affine MVP list of the current block (S1420). Theconstructed candidate may represent the above-described constructedaffine candidate. In case a number of available inherited candidates isless than 2, the encoding apparatus/decoding apparatus may add aconstructed candidate to an affine MVP list of the current block.

The encoding apparatus/decoding apparatus may derive mv₀ from a firstgroup, mv₁ from a second group, and mv₂ from a third group, and theencoding apparatus/decoding apparatus may derive the constructed affinecandidate including the mv₀, the mv₁, and the mv₂. The mv₀ may be aCPMVP candidate of CP0, the mv₁ may be a CPMVP candidate of CP1, and themv₂ may be a CPMVP candidate of CP2.

Herein, a reference picture for the mv₀ may be the same as a referencepicture of the current block. More specifically, when checking motionvectors within the first group according to a specific order, the mv₀may be a first motion vector that is verified to be satisfying acondition. The condition may be that a reference picture for a motionvector is the same as a reference picture of the current block.Additionally, a reference picture for the mv₁ may be the same as areference picture of the current block. More specifically, when checkingmotion vectors within the second group according to a specific order,the mv₁ may be a first motion vector that is verified to be satisfying acondition. The condition may be that a reference picture for a motionvector is the same as a reference picture of the current block.Additionally, a reference picture for the mv₂ may be the same as areference picture of the current block. More specifically, when checkingmotion vectors within the third group according to a specific order, themv₂ may be a first motion vector that is verified to be satisfying acondition. The condition may be that a reference picture for a motionvector is the same as a reference picture of the current block.

Additionally, the first group may include a motion vector of theneighboring block A, a motion vector of the neighboring block B, and amotion vector of the neighboring block C. The second group may include amotion vector of the neighboring block D and a motion vector of theneighboring block E. The third group may include a motion vector of theneighboring block F and a motion vector of the neighboring block G. Theneighboring block A may denote a neighboring block positioned at atop-left of a top-left sample position of the current block, theneighboring block B may denote a neighboring block positioned at a topof the top-left sample position of the current block, and theneighboring block C may denote a neighboring block positioned at aleft-side of the top-left sample position of the current block.Additionally, the neighboring block D may denote a neighboring blockpositioned at a top of a top-right sample position of the current block,and the neighboring block E may denote a neighboring block positioned ata top-right of the top-right sample position of the current block.Additionally, the neighboring block F may denote a neighboring blockpositioned at a left side of a bottom-left sample position of thecurrent block, and the neighboring block G may denote a neighboringblock positioned at a bottom-left of the bottom-left sample position ofthe current block.

The encoding apparatus/decoding apparatus may add an HEVC AMVP candidateto the affine MVP list of the current block (S1430). In case a number ofavailable inherited candidates and/or constructed candidates is lessthan 2, the encoding apparatus/decoding apparatus may add an HEVC AMVPcandidate to the affine MVP list of the current block. Morespecifically, in case the number of available inherited candidatesand/or constructed candidates is less than 2, the encodingapparatus/decoding apparatus may perform a process of configuring MVPcandidates of the existing HEVC standard.

FIG. 15 illustrates an example for constructing an affine MVP list.

Referring to FIG. 15, an encoding apparatus/decoding apparatus may addan inherited candidate to an affine MVP list of a current block (S1500).The inherited candidate may represent the above-described inheritedaffine candidate.

More specifically, the encoding apparatus/decoding apparatus may derivea maximum of 1 inherited affine candidate from neighboring blocks of thecurrent block (S1505). Herein, the neighboring blocks may include a leftneighboring block A0 of the current block, a bottom-left cornerneighboring block A1 of the current block, a top neighboring block B0 ofthe current block, a top-right corner neighboring block B1 of thecurrent block, and a top-left corner neighboring block B2 of the currentblock.

For example, the encoding apparatus/decoding apparatus may derive afirst affine MVP candidate based on a first block within the neighboringblocks. Herein, the first block may be coded by using an affine motionmodel, and a reference picture of the first block and a referencepicture of the current block may be the same. More specifically, whenchecking the neighboring blocks by a specific order, the first block maybe a first block that is verified to be satisfying the conditions. Theconditions may be that coding is performed by using the affine motionmodel, and that a reference picture of the block is the same as thereference picture of the current block.

Meanwhile, the specific order may be left neighboring blockA0→bottom-left corner neighboring block A1→top neighboring blockB0→top-right corner neighboring block B1→top-left corner neighboringblock B2. Additionally, the checking may be performed by an order otherthan the above-described order and may not be limited only to theabove-described example.

The encoding apparatus/decoding apparatus may add a constructedcandidate to an affine MVP list of the current block (S1510). Theconstructed candidate may represent the above-described constructedaffine candidate. In case a number of available inherited candidates isless than 2, the encoding apparatus/decoding apparatus may add aconstructed candidate to an affine MVP list of the current block.

The encoding apparatus/decoding apparatus may derive mv₀ from a firstgroup, mv₁ from a second group, and mv₂ from a third group, and theencoding apparatus/decoding apparatus may derive the constructed affinecandidate including the mv₀, the mv₁, and the mv₂. The mv₀ may be aCPMVP candidate of CP0, the mv₁ may be a CPMVP candidate of CP1, and themv₂ may be a CPMVP candidate of CP2.

Herein, a reference picture for the mv₀ may be the same as a referencepicture of the current block. More specifically, when checking motionvectors within the first group according to a specific order, the mv₀may be a first motion vector that is verified to be satisfying acondition. The condition may be that a reference picture for a motionvector is the same as a reference picture of the current block.Additionally, a reference picture for the mv₁ may be the same as areference picture of the current block. More specifically, when checkingmotion vectors within the second group according to a specific order,the mv₁ may be a first motion vector that is verified to be satisfying acondition. The condition may be that a reference picture for a motionvector is the same as a reference picture of the current block.Additionally, a reference picture for the mv₂ may be the same as areference picture of the current block. More specifically, when checkingmotion vectors within the third group according to a specific order, themv₂ may be a first motion vector that is verified to be satisfying acondition. The condition may be that a reference picture for a motionvector is the same as a reference picture of the current block.

Additionally, the first group may include a motion vector of theneighboring block A, a motion vector of the neighboring block B, and amotion vector of the neighboring block C. The second group may include amotion vector of the neighboring block D and a motion vector of theneighboring block E. The third group may include a motion vector of theneighboring block F and a motion vector of the neighboring block G. Theneighboring block A may denote a neighboring block positioned at atop-left of a top-left sample position of the current block, theneighboring block B may denote a neighboring block positioned at a topof the top-left sample position of the current block, and theneighboring block C may denote a neighboring block positioned at aleft-side of the top-left sample position of the current block.Additionally, the neighboring block D may denote a neighboring blockpositioned at a top of a top-right sample position of the current block,and the neighboring block E may denote a neighboring block positioned ata top-right of the top-right sample position of the current block.Additionally, the neighboring block F may denote a neighboring blockpositioned at a left side of a bottom-left sample position of thecurrent block, and the neighboring block G may denote a neighboringblock positioned at a bottom-left of the bottom-left sample position ofthe current block.

The encoding apparatus/decoding apparatus may add an HEVC AMVP candidateto the affine MVP list of the current block (S1520). In case a number ofavailable inherited candidates and/or constructed candidates is lessthan 2, the encoding apparatus/decoding apparatus may add an HEVC AMVPcandidate to the affine MVP list of the current block. Morespecifically, in case the number of available inherited candidatesand/or constructed candidates is less than 2, the encodingapparatus/decoding apparatus may perform a process of configuring MVPcandidates of the existing HEVC standard.

The embodiments disclosed in the above-described flow charts havedifferences in the process of deriving the inherited affinecandidate(s). Therefore, comparison may be made between theabove-described embodiments by performing a complexity analysis of theprocess of deriving the inherited affine candidate(s).

The complexity analysis of the above-described embodiments may bederived as shown below in the following table.

TABLE 7 Inherited affine AMVP candidates max max number number Length ofmotion of max number max max of AMVP vector potential max number of maxnumber max number number of number of max number candidate scalingcandidate of derived comparison of shift of mult. div. add of absolutelist operation positions candidates operation operation operationoperation operation operation FIG. 13 2 0 5 10 36 100 0 0 100 0 FIG. 142 0 5 2 4 20 0 0 20 0 FIG. 15 2 0 5 1 0 10 0 0 10 0

Additionally, the coding performance of the above-described embodimentsmay be derived as shown below in the following table.

TABLE 8 VTM-2.0 over VTM1.0 + Affine + High Precision MV Y U V EncT DecTClass A1 −0.04% −0.11% −0.01% 101% 100% Class A2 −0.51% −0.29% −0.49% 99% 100% Class B −0.28% −0.03% −0.08% 100%  99% Class C −0.01% −0.11%−0.13%  98%  93% Overall −0.21% −0.12% −0.16% 100%  98% Class D −0.01%−0.23% −0.10% 101% 101% Proposed (Section 2.3) over VTM1.0 + Affine +High Precision MV Y U V EncT DecT Class A1 −0.02% −0.10% 0.19% 100% 101%Class A2 −0.49% −0.44% −0.38% 100% 101% Class B −0.31% 0.03% −0.16% 100% 98% Class C −0.07% −0.04% −0.07% 100% 100% Overall −0.22% −0.11% −0.11%100% 100% Class D −0.05% −0.18% −0.05% 101% 103% Proposed (Section 2.4)over VTM1.0 + Affine + High Precision MV Y U V EncT DecT Class A1 −0.07%−0.14% 0.07% 99% 99% Class A2 −0.50% −0.47% −0.42% 99% 100%  Class B−0.26% 0.07% −0.17% 100%  98% Class C −0.10% −0.13% 0.05% 99% 95%Overall −0.23% −0.13% −0.11% 99% 98% Class D −0.03% −0.34% 0.15% 100% 101% 

Referring to Table 7, it may be verified that the embodiment of FIG. 14and the embodiment of FIG. 15 have lower complexity than the embodimentof FIG. 13. Additionally, referring to Table 8, it may be verified thatcoding performances of the embodiment of FIG. 13, the embodiment of FIG.14, and the embodiment of FIG. 15 are almost the same. Therefore, theembodiment of FIG. 14 and the embodiment of FIG. 15, which have lowercomplexity and the same coding performance, may be considered more forthe encoding/decoding process.

FIG. 16 illustrates a general view of an image encoding method performedby an encoding apparatus according to the present disclosure. The methodproposed in FIG. 16 may be performed by the encoding apparatus (ordevice) disclosed in FIG. 1. More specifically, for example, steps S1600to S1630 of FIG. 16 may be performed by a predictor of the encodingapparatus, and step S1640 may be performed by an entropy encoder of theencoding apparatus. Additionally, although it is not shown in thedrawing, a process of deriving prediction samples for the current blockbased on the CPMVs may be performed by a predictor of the encodingapparatus. A process of deriving a residual sample for the current blockbased on an original sample and prediction sample for the current blockmay be performed by a subtractor of the encoding apparatus. A process ofgenerating information related to a residual for the current block basedon the residual sample may be performed by a transformer of the encodingapparatus. And, a process of encoding the information related to theresidual may be performed by an entropy encoder of the encodingapparatus.

The encoding apparatus constructs an affine MVP candidate list includingaffine Motion Vector Predictor (MVP) candidates for the current block(S1600).

For example, the affine MVP candidates may include a first affine MVPcandidate and a second affine MVP candidate.

The first affine MVP candidate may be derived based on a first blockwithin a left block group, which includes a bottom-left cornerneighboring block and a left neighboring block of the current block.Herein, the first block may be coded by using an affine motion model,and a reference picture of the first block and a reference picture ofthe current block may be the same.

More specifically, the first block may be a first block that is verifiedto be satisfying the conditions, when checking neighboring blocks withinthe left block group according to a specific order. The conditions maybe that coding is performed by using an affine motion model, and that areference picture of a block is the same as a reference picture of thecurrent block. For example, the encoding apparatus may check accordingto the specific order whether or not blocks within the left block groupsatisfy the conditions, the encoding apparatus may then derive a firstblock being the first to satisfy the conditions and may derive the firstaffine MVP candidate based on the first block.

More specifically, for example, the encoding apparatus may derive motionvectors for CPs of the current block based on an affine motion model ofthe first block and may derive the first affine MVP candidate includingthe motion vectors as CPMVP candidates. The affine motion model may bederived as shown in Equation 1 or Equation 3, which are presented above.

Meanwhile, the specific order for checking blocks within the left blockgroup may be an order starting from the left neighboring block to thebottom-left corner neighboring block. Alternatively, the specific orderfor checking the left block group may be an order starting from thebottom-left corner neighboring block to the left neighboring block.

The second affine MVP candidate may be derived based on a second blockwithin a top block group, which includes a top-right corner neighboringblock, a top neighboring block, and a top-left corner neighboring blockof the current block. Herein, the second block may be coded by using anaffine motion model, and a reference picture of the second block and areference picture of the current block may be the same.

More specifically, the second block may be a first block that isverified to be satisfying the conditions, when checking neighboringblocks within the top block group according to a specific order. Theconditions may be that coding is performed by using an affine motionmodel, and that a reference picture of a block is the same as areference picture of the current block. For example, the encodingapparatus may check according to the specific order whether or notblocks within the top block group satisfy the conditions, the encodingapparatus may then derive a second block being the first to satisfy theconditions and may derive the second affine MVP candidate based on thesecond block.

More specifically, for example, the encoding apparatus may derive motionvectors for CPs of the current block based on an affine motion model ofthe second block and may derive the second affine MVP candidateincluding the motion vectors as CPMVP candidates. The affine motionmodel may be derived as shown in Equation 1 or Equation 3, which arepresented above.

Meanwhile, the specific order for checking blocks within the top blockgroup may be an order starting from the top neighboring block to thetop-right corner neighboring block and the top-left corner neighboringblock.

Additionally, as another example, the affine MVP candidates may includea first affine MVP candidate, which is derived as described below.

The first affine MVP candidate may be derived based on a first blockwithin neighboring blocks of the current block. Herein, the first blockmay be coded by using an affine motion model, and a reference picture ofthe first block and a reference picture of the current block may be thesame. Additionally, the neighboring blocks may include the leftneighboring block, the bottom-left corner neighboring block, thetop-right corner neighboring block, the top neighboring block, and thetop-left corner neighboring block of the current block.

The first block may be a first block that is verified to be satisfyingthe conditions, when checking the neighboring blocks according to aspecific order. The conditions may be that coding is performed by usingan affine motion model, and that a reference picture of a block is thesame as a reference picture of the current block. For example, theencoding apparatus may check according to the specific order whether ornot the neighboring blocks satisfy the conditions, the encodingapparatus may then derive a first block being the first to satisfy theconditions and may derive the first affine MVP candidate based on thefirst block.

More specifically, for example, the encoding apparatus may derive motionvectors for CPs of the current block based on an affine motion model ofthe first block and may derive the first affine MVP candidate includingthe motion vectors as CPMVP candidates. The affine motion model may bederived as shown in Equation 1 or Equation 3, which are presented above.

Meanwhile, the specific order for checking the neighboring blocks may bean order starting from the left neighboring block to the bottom-leftcorner neighboring block, the top neighboring block, the top-rightcorner neighboring block, and the top-left corner neighboring block.

Herein, for example, in case the size of the current block is W×H, and,in case, the x component of the top-left sample position of the currentblock is 0 and the y component is 0, the left neighboring block may be ablock including a sample of coordinates (−1, H−1), the top neighboringblock may be a block including a sample of coordinates (W−1, −1), thetop-right neighboring block may be a block including a sample ofcoordinates (W, −1), the bottom-left neighboring block may be a blockincluding a sample of coordinates (−1, H), and the top-left neighboringblock may be a block including a sample of coordinates (−1, −1).

Meanwhile, in case the first affine MVP candidate and/or the secondaffine MVP candidate are/is not derived, i.e., in case a number ofaffine MVP candidates less than 2 are/is derived by performing theabove-described process, the affine MVP candidates may include aconstructed affine MVP candidate.

In other words, for example, in case the first affine MVP candidateand/or the second affine MVP candidate are/is not derived, i.e., in casea number of affine MVP candidates less than 2 are/is derived byperforming the above-described process, the encoding apparatus mayderive a constructed affine MVP candidate based on the neighboringblocks.

More specifically, the encoding apparatus may divide the motion vectorsof the neighboring blocks into a first group, a second group, and athird group. The first group may include a motion vector of neighboringblock A, a motion vector of neighboring block B, and a motion vector ofneighboring block C. The second group may include a motion vector ofneighboring block D and a motion vector of neighboring block E. And, thethird group may include a motion vector of neighboring block F and amotion vector of neighboring block G.

Additionally, the encoding apparatus may derive a CPMVP candidate forCP0 of the current block from the first group, a CPMVP candidate for CP1of the current block from the second group, and a CPMVP candidate forCP2 of the current block from the third group. And, the encodingapparatus may derive the constructed affine MVP candidate including theCPMVP candidates for the CPs.

The CPMVP candidate for CP0 may be a first motion vector that isverified to be satisfying conditions, when checking motion vectorswithin the first group according to a specific order. The condition maybe that a reference picture for a motion vector is the same as areference picture of the current block. In other words, the CPMVPcandidate for CP0 may be a motion vector having a reference picture thatis first verified to be the same as a reference picture of the currentblock, when checking motion vectors within the first group according toa specific order. For example, the encoding apparatus may check motionvectors within the first group according to the specific order to verifyif the motion vectors satisfy the condition, and the first motion vectorthat is verified to be satisfying the condition may be derived as theCPMVP candidate for CP0. Herein, for example, the specific order may bean order starting from the neighboring block A to the neighboring blockB and the neighboring block C.

The CPMVP candidate for CP1 may be a first motion vector that isverified to be satisfying conditions, when checking motion vectorswithin the second group according to a specific order. The condition maybe that a reference picture for a motion vector is the same as areference picture of the current block. In other words, the CPMVPcandidate for CP1 may be a motion vector having a reference picture thatis first verified to be the same as a reference picture of the currentblock, when checking motion vectors within the second group according toa specific order. For example, the encoding apparatus may check motionvectors within the second group according to the specific order toverify if the motion vectors satisfy the condition, and the first motionvector that is verified to be satisfying the condition may be derived asthe CPMVP candidate for CP1. Herein, for example, the specific order maybe an order starting from the neighboring block D to the neighboringblock E.

The CPMVP candidate for CP2 may be a first motion vector that isverified to be satisfying conditions, when checking motion vectorswithin the third group according to a specific order. The condition maybe that a reference picture for a motion vector is the same as areference picture of the current block. In other words, the CPMVPcandidate for CP2 may be a motion vector having a reference picture thatis first verified to be the same as a reference picture of the currentblock, when checking motion vectors within the third group according toa specific order. For example, the encoding apparatus may check motionvectors within the third group according to the specific order to verifyif the motion vectors satisfy the condition, and the first motion vectorthat is verified to be satisfying the condition may be derived as theCPMVP candidate for CP2. Herein, for example, the specific order may bean order starting from the neighboring block F to the neighboring blockG.

Herein, the neighboring block A may represent a neighboring blockpositioned at the top-left of a top-left sample position of the currentblock, the neighboring block B may represent a neighboring blockpositioned at the top of the top-left sample position of the currentblock, and the neighboring block C may represent a neighboring blockpositioned at a left-side of the top-left sample position of the currentblock, the neighboring block D may represent a neighboring blockpositioned at the top of a top-right sample position of the currentblock, the neighboring block E may represent a neighboring blockpositioned at the top-right of the top-right sample position of thecurrent block, the neighboring block F may represent a neighboring blockpositioned at a left-side of a bottom-left sample position of thecurrent block, and the neighboring block G may represent a neighboringblock positioned at the bottom-left of the bottom-left sample positionof the current block.

Herein, for example, in case the size of the current block is W×H, and,in case, the x component of the top-left sample position of the currentblock is 0 and the y component is 0, the neighboring block A may be ablock including a sample of coordinates (−1, −1), the neighboring blockB may be a block including a sample of coordinates (0, −1), theneighboring block C may be a block including a sample of coordinates(−1, 0), the neighboring block D may be a block including a sample ofcoordinates (W−1, −1), the neighboring block E may be a block includinga sample of coordinates (W, −1), the neighboring block F may be a blockincluding a sample of coordinates (−1, H−1), and the neighboring block Gmay be a block including a sample of coordinates (−1, H). Additionally,the CP0 may denote a top-left position of the current block, the CP1 maydenote a top-right position of the current block, and the CP2 may denotea bottom-left position of the current block. In other words, the CPs mayinclude CP0 being positioned at a top-left sample position of thecurrent block and CP1 being positioned at a top-right sample position ofthe current block, and the CPs may further include CP2 being positionedat a bottom-left sample position of the current block.

Meanwhile, in case a CPMVP candidate for CP0 of the current block isderived from the first group, in case a CPMVP candidate for CP1 of thecurrent block is derived from the second group, and a CPMVP candidatefor CP2 of the current block is not derived from the third group, theCPMVP candidate for CP2 may be derived by using the above-describedEquation 8 based on the CPMVP candidate for CP0 and the CPMVP candidatefor CP1.

Additionally, in case a CPMVP candidate for CP0 of the current block isderived from the first group, in case a CPMVP candidate for CP2 of thecurrent block is derived from the third group, and a CPMVP candidate forCP1 of the current block is not derived from the second group, the CPMVPcandidate for CP1 may be derived by using the above-described Equation 9based on the CPMVP candidate for CP0 and the CPMVP candidate for CP2.

Meanwhile, in case a number of affine MVP candidates less than 2 are/isderived by performing the above-described process, the affine MVPcandidates may include MVP candidates of the existing HEVC standard.

In other words, for example, in case less than 2 affine MVP candidatesare/is derived by performing the above-described process, the encodingapparatus may derive MVP candidates of the existing HEVC standard.

The encoding apparatus derives Control Point Motion Vector Predictors(CPMVPs) for Control Points (CPs) of the current block based on oneaffine MVP candidate among the affine MVP candidates included in theaffine MVP candidate list (S1610). The encoding apparatus may deriveCPMVs for the CPs of the current block having an optimal RD cost and mayselect an affine MVP candidate most similar to the CPMVs, among theaffine MVP candidates, as the affine MVP candidate for the currentblock. Based on the selected affine MVP candidate, which is selectedfrom the affine MVP candidate list, the encoding apparatus may deriveControl Point Motion Vector Predictors (CPMVPs) for Control Points (CPs)of the current block.

The encoding apparatus may encode an affine MVP candidate indexindicating the selected affine MVP candidate among the affine MVPcandidates. The affine MVP candidate index may indicate the one affineMVP candidate, which is selected from the affine MVP candidates beingincluded in the affine Motion Vector Predictor (MVP) candidate list forthe current block.

The encoding apparatus derives CPMVs for the CPs of the current block(S1620). The encoding apparatus may derive CPMVs for each of the CPs ofthe current block.

The encoding apparatus derives Control Point Motion Vector Differences(CPMVDs) for the CPs of the current block based on the CPMVPs and theCPMVs (S1630). The encoding apparatus may derive CPMVPs for the CPs ofthe current block based on the CPMVPs and the CPMVs for each of the CPs.

The encoding apparatus encodes motion prediction information includinginformation on the CPMVDs (S1640). The encoding apparatus may output themotion prediction information including information on the CPMVDs in abitstream format. In other words, the encoding apparatus may outputimage (or video) information including the motion prediction informationin a bitstream format. The encoding apparatus may encode information onthe CPMVDs for each of the CPs, and the motion prediction informationmay include information on the CPMVDs.

Additionally, the motion prediction information may include the affineMVP candidate index. The affine MVP candidate index may indicate theselected affine MVP candidate, which is selected from the affine MVPcandidates being included in the affine Motion Vector Predictor (MVP)candidate list for the current block.

Meanwhile, for example, the encoding apparatus may derive predictionsamples for the current block based on the CPMVs and may derive aresidual sample for the current block based on an original sample andprediction sample for the current block. Then, the encoding apparatusmay generate information on a residual for the current block may begenerated based on the residual sample and may encode the information onthe residual. The image information may include the information on theresidual.

Meanwhile, the bitstream may be transmitted to a decoding apparatusthrough a network or a (digital) storage medium. Herein, the network mayinclude a broadcast network and/or a communication network, and so on,and the digital storage medium may include various storage media, suchas USB, SD, CD, DVD, Bluray, HDD, SSD, and so on.

FIG. 17 illustrates a general view of an encoding apparatus performingan image encoding method according to the present disclosure. The methoddisclosed in FIG. 16 may be performed by the encoding apparatusdisclosed in FIG. 17. More specifically, for example, a predictor of theencoding apparatus of FIG. 17 may perform steps S1600 to S1630 of FIG.16, and an entropy encoder of the encoding apparatus of FIG. 17 mayperform step S1640 of FIG. 16. Additionally, although it is not shown inthe drawing, a process of deriving prediction samples for the currentblock based on the CPMVs may be performed by a predictor of the encodingapparatus of FIG. 17. A process of deriving a residual sample for thecurrent block based on an original sample and prediction sample for thecurrent block may be performed by a subtractor of the encoding apparatusof FIG. 17. A process of generating information related to a residualfor the current block based on the residual sample may be performed by atransformer of the encoding apparatus of FIG. 17. And, a process ofencoding the information related to the residual may be performed by anentropy encoder of the encoding apparatus of FIG. 17.

FIG. 18 illustrates a general view of an image decoding method performedby a decoding apparatus according to the present disclosure. The methodproposed in FIG. 18 may be performed by the decoding apparatus (ordevice) disclosed in FIG. 2. More specifically, for example, steps S1810to S1850 of FIG. 18 may be performed by a predictor of the decodingapparatus, and step S1860 may be performed by an adder of the decodingapparatus. Additionally, although it is not shown in the drawing, aprocess of obtaining information on a residual for the current blockthrough a bitstream may be performed by an entropy decoder of thedecoding apparatus, and a process of deriving the residual sample forthe current block based on the residual information may be performed byan inverse transformer of the decoding apparatus.

The decoding apparatus may obtain motion prediction information from abitstream (S1800). The decoding apparatus may obtain image (or video)information including the motion prediction information from thebitstream.

Additionally, for example, the motion prediction information may includeinformation on Control Point Motion Vector Differences (CPMVDs) forcontrol points (CPs) of the current block. In other words, the motionprediction information may include information on the CPMVDs for each ofthe CPs of the current block.

Additionally, for example, the motion prediction information may includean affine MVP candidate index for the current block. The affine MVPcandidate index may indicate one of the affine MVP candidates includedin an affine Motion Vector Predictor (MVP) candidate list for thecurrent block.

The decoding apparatus constructs an affine MVP candidate list includingaffine Motion Vector Predictor (MVP) candidates for the current block(S1810).

For example, the affine MVP candidates may include a first affine MVPcandidate and a second affine MVP candidate.

The first affine MVP candidate may be derived based on a first blockwithin a left block group, which includes a bottom-left cornerneighboring block and a left neighboring block of the current block.Herein, the first block may be coded by using an affine motion model,and a reference picture of the first block and a reference picture ofthe current block may be the same.

More specifically, the first block may be a first block that is verifiedto be satisfying the conditions, when checking neighboring blocks withinthe left block group according to a specific order. The conditions maybe that coding is performed by using an affine motion model, and that areference picture of a block is the same as a reference picture of thecurrent block. For example, the decoding apparatus may check accordingto the specific order whether or not blocks within the left block groupsatisfy the conditions, the decoding apparatus may then derive a firstblock being the first to satisfy the conditions and may derive the firstaffine MVP candidate based on the first block.

More specifically, for example, the decoding apparatus may derive motionvectors for CPs of the current block based on an affine motion model ofthe first block and may derive the first affine MVP candidate includingthe motion vectors as CPMVP candidates. The affine motion model may bederived as shown in Equation 1 or Equation 3, which are presented above.

Meanwhile, the specific order for checking blocks within the left blockgroup may be an order starting from the left neighboring block to thebottom-left corner neighboring block. Alternatively, the specific orderfor checking the left block group may be an order starting from thebottom-left corner neighboring block to the left neighboring block.

The second affine MVP candidate may be derived based on a second blockwithin a top block group, which includes a top-right corner neighboringblock, a top neighboring block, and a top-left corner neighboring blockof the current block. Herein, the second block may be coded by using anaffine motion model, and a reference picture of the second block and areference picture of the current block may be the same.

More specifically, the second block may be a first block that isverified to be satisfying the conditions, when checking neighboringblocks within the top block group according to a specific order. Theconditions may be that coding is performed by using an affine motionmodel, and that a reference picture of a block is the same as areference picture of the current block. For example, the decodingapparatus may check according to the specific order whether or notblocks within the top block group satisfy the conditions, the decodingapparatus may then derive a second block being the first to satisfy theconditions and may derive the second affine MVP candidate based on thesecond block.

More specifically, for example, the decoding apparatus may derive motionvectors for CPs of the current block based on an affine motion model ofthe second block and may derive the second affine MVP candidateincluding the motion vectors as CPMVP candidates. The affine motionmodel may be derived as shown in Equation 1 or Equation 3, which arepresented above.

Meanwhile, the specific order for checking blocks within the top blockgroup may be an order starting from the top neighboring block to thetop-right corner neighboring block and the top-left corner neighboringblock.

Additionally, as another example, the affine MVP candidates may includea first affine MVP candidate, which is derived as described below.

The first affine MVP candidate may be derived based on a first blockwithin neighboring blocks of the current block. Herein, the first blockmay be coded by using an affine motion model, and a reference picture ofthe first block and a reference picture of the current block may be thesame. Additionally, the neighboring blocks may include the leftneighboring block, the bottom-left corner neighboring block, thetop-right corner neighboring block, the top neighboring block, and thetop-left corner neighboring block of the current block.

The first block may be a first block that is verified to be satisfyingthe conditions, when checking the neighboring blocks according to aspecific order. The conditions may be that coding is performed by usingan affine motion model, and that a reference picture of a block is thesame as a reference picture of the current block. For example, thedecoding apparatus may check according to the specific order whether ornot the neighboring blocks satisfy the conditions, the decodingapparatus may then derive a first block being the first to satisfy theconditions and may derive the first affine MVP candidate based on thefirst block.

More specifically, for example, the decoding apparatus may derive motionvectors for CPs of the current block based on an affine motion model ofthe first block and may derive the first affine MVP candidate includingthe motion vectors as CPMVP candidates. The affine motion model may bederived as shown in Equation 1 or Equation 3, which are presented above.

Meanwhile, the specific order for checking the neighboring blocks may bean order starting from the left neighboring block to the bottom-leftcorner neighboring block, the top neighboring block, the top-rightcorner neighboring block, and the top-left corner neighboring block.

Herein, for example, in case the size of the current block is W×H, and,in case, the x component of the top-left sample position of the currentblock is 0 and the y component is 0, the left neighboring block may be ablock including a sample of coordinates (−1, H−1), the top neighboringblock may be a block including a sample of coordinates (W−1, −1), thetop-right neighboring block may be a block including a sample ofcoordinates (W, −1), the bottom-left neighboring block may be a blockincluding a sample of coordinates (−1, H), and the top-left neighboringblock may be a block including a sample of coordinates (−1, −1).

Meanwhile, in case the first affine MVP candidate and/or the secondaffine MVP candidate are/is not derived, i.e., in case a number ofaffine MVP candidates less than 2 are/is derived by performing theabove-described process, the affine MVP candidates may include aconstructed affine MVP candidate.

In other words, for example, in case the first affine MVP candidateand/or the second affine MVP candidate are/is not derived, i.e., in casea number of affine MVP candidates less than 2 are/is derived byperforming the above-described process, the decoding apparatus mayderive a constructed affine MVP candidate based on the neighboringblocks.

More specifically, the decoding apparatus may divide the motion vectorsof the neighboring blocks into a first group, a second group, and athird group. The first group may include a motion vector of neighboringblock A, a motion vector of neighboring block B, and a motion vector ofneighboring block C. The second group may include a motion vector ofneighboring block D and a motion vector of neighboring block E. And, thethird group may include a motion vector of neighboring block F and amotion vector of neighboring block G.

Additionally, the decoding apparatus may derive a CPMVP candidate forCP0 of the current block from the first group, a CPMVP candidate for CP1of the current block from the second group, and a CPMVP candidate forCP2 of the current block from the third group. And, the decodingapparatus may derive the constructed affine MVP candidate including theCPMVP candidates for the CPs.

The CPMVP candidate for CP0 may be a first motion vector that isverified to be satisfying conditions, when checking motion vectorswithin the first group according to a specific order. The condition maybe that a reference picture for a motion vector is the same as areference picture of the current block. In other words, the CPMVPcandidate for CP0 may be a motion vector having a reference picture thatis first verified to be the same as a reference picture of the currentblock, when checking motion vectors within the first group according toa specific order. For example, the decoding apparatus may check motionvectors within the first group according to the specific order to verifyif the motion vectors satisfy the condition, and the first motion vectorthat is verified to be satisfying the condition may be derived as theCPMVP candidate for CP0. Herein, for example, the specific order may bean order starting from the neighboring block A to the neighboring blockB and the neighboring block C.

The CPMVP candidate for CP1 may be a first motion vector that isverified to be satisfying conditions, when checking motion vectorswithin the second group according to a specific order. The condition maybe that a reference picture for a motion vector is the same as areference picture of the current block. In other words, the CPMVPcandidate for CP1 may be a motion vector having a reference picture thatis first verified to be the same as a reference picture of the currentblock, when checking motion vectors within the second group according toa specific order. For example, the decoding apparatus may check motionvectors within the second group according to the specific order toverify if the motion vectors satisfy the condition, and the first motionvector that is verified to be satisfying the condition may be derived asthe CPMVP candidate for CP1. Herein, for example, the specific order maybe an order starting from the neighboring block D to the neighboringblock E.

The CPMVP candidate for CP2 may be a first motion vector that isverified to be satisfying conditions, when checking motion vectorswithin the third group according to a specific order. The condition maybe that a reference picture for a motion vector is the same as areference picture of the current block. In other words, the CPMVPcandidate for CP2 may be a motion vector having a reference picture thatis first verified to be the same as a reference picture of the currentblock, when checking motion vectors within the third group according toa specific order. For example, the decoding apparatus may check motionvectors within the third group according to the specific order to verifyif the motion vectors satisfy the condition, and the first motion vectorthat is verified to be satisfying the condition may be derived as theCPMVP candidate for CP2. Herein, for example, the specific order may bean order starting from the neighboring block F to the neighboring blockG.

Herein, the neighboring block A may represent a neighboring blockpositioned at the top-left of a top-left sample position of the currentblock, the neighboring block B may represent a neighboring blockpositioned at the top of the top-left sample position of the currentblock, and the neighboring block C may represent a neighboring blockpositioned at a left-side of the top-left sample position of the currentblock, the neighboring block D may represent a neighboring blockpositioned at the top of a top-right sample position of the currentblock, the neighboring block E may represent a neighboring blockpositioned at the top-right of the top-right sample position of thecurrent block, the neighboring block F may represent a neighboring blockpositioned at a left-side of a bottom-left sample position of thecurrent block, and the neighboring block G may represent a neighboringblock positioned at the bottom-left of the bottom-left sample positionof the current block.

Herein, for example, in case the size of the current block is W×H, and,in case, the x component of the top-left sample position of the currentblock is 0 and the y component is 0, the neighboring block A may be ablock including a sample of coordinates (−1, −1), the neighboring blockB may be a block including a sample of coordinates (0, −1), theneighboring block C may be a block including a sample of coordinates(−1, 0), the neighboring block D may be a block including a sample ofcoordinates (W−1, −1), the neighboring block E may be a block includinga sample of coordinates (W, −1), the neighboring block F may be a blockincluding a sample of coordinates (−1, H−1), and the neighboring block Gmay be a block including a sample of coordinates (−1, H). Additionally,the CP0 may denote a top-left position of the current block, the CP1 maydenote a top-right position of the current block, and the CP2 may denotea bottom-left position of the current block. In other words, the CPs mayinclude CP0 being positioned at a top-left sample position of thecurrent block and CP1 being positioned at a top-right sample position ofthe current block, and the CPs may further include CP2 being positionedat a bottom-left sample position of the current block.

Meanwhile, in case a CPMVP candidate for CP0 of the current block isderived from the first group, in case a CPMVP candidate for CP1 of thecurrent block is derived from the second group, and a CPMVP candidatefor CP2 of the current block is not derived from the third group, theCPMVP candidate for CP2 may be derived by using the above-describedEquation 8 based on the CPMVP candidate for CP0 and the CPMVP candidatefor CP1.

Additionally, in case a CPMVP candidate for CP0 of the current block isderived from the first group, in case a CPMVP candidate for CP2 of thecurrent block is derived from the third group, and a CPMVP candidate forCP1 of the current block is not derived from the second group, the CPMVPcandidate for CP1 may be derived by using the above-described Equation 9based on the CPMVP candidate for CP0 and the CPMVP candidate for CP2.

Meanwhile, in case a number of affine MVP candidates less than 2 are/isderived by performing the above-described process, the affine MVPcandidates may include MVP candidates of the existing HEVC standard.

In other words, for example, in case less than 2 affine MVP candidatesare/is derived by performing the above-described process, the decodingapparatus may derive MVP candidates of the existing HEVC standard.

The decoding apparatus derives Control Point Motion Vector Predictors(CPMVPs) for Control Points (CPs) of the current block based on oneaffine MVP candidate among the affine MVP candidates included in theaffine MVP candidate list (S1820).

The decoding apparatus may select a specific affine MVP candidate fromthe affine MVP candidates included in the affine MVP candidate list, andthe decoding apparatus may then derive the selected affine MVP candidateas CPMVPs for the CPs of the current block. For example, the decodingapparatus may obtain the affine MVP candidate index for the currentblock from the bitstream, and, then, the decoding apparatus may derivean affine MVP candidate, which is indicated by the affine MVP candidateindex, among the affine MVP candidates including the affine MVPcandidate list, as CPMVPs for the CPs of the current block.

The decoding apparatus derives Control Point Motion Vector Differences(CPMVDs) for the CPs of the current block based on the motion predictioninformation (S1830). The motion prediction information may includeinformation on CPMVDs for each of the CPs, and the decoding apparatusmay derive the CPMVDs for each of the CPs of the current block based onthe information on the CPMVDs for each of the CPs.

The decoding apparatus derives Control Point Motion Vectors (CPMVs) forthe CPs of the current block based on the CPMVPs and the CPMVDs (S1840).The decoding apparatus may derive the CPMVs for each CP based on theCPMVPs and the CPMVDs for each of the CPs. For example, the decodingapparatus may derive the CPMV for the CP by adding the CPMVPs and theCPMVDs for each CP.

The decoding apparatus derives prediction samples for the current blockbased on the CPMVs (S1850). The decoding apparatus may derive sub-blockunits or sample units of motion vectors of the current block based onthe CPMVs. That is, the decoding apparatus may derive the motion vectorof each sub-block or each sample of the current block. The sub-blockunit or sample unit motion vectors may be derived based on Equation 1 orEquation 3, which are described above. The motion vectors may beindicated as a Motion Vector Field (MVF) or a motion vector array.

The decoding apparatus may derive prediction samples for the currentblock based on the sub-block unit or sample unit motion vectors. Thedecoding apparatus may derive a reference area within a referencepicture based on the sub-block unit or sample unit motion vectors andmay generate a prediction sample of the current block based on areconstructed sample within the reference area.

The decoding apparatus generates a reconstructed picture for the currentblock based on the derived prediction samples (S1860). The decodingapparatus may generate a reconstructed picture for the current blockbased on the derived prediction samples. The decoding apparatus mayimmediately use a prediction sample as a reconstructed sample inaccordance with the prediction mode, or the decoding apparatus maygenerate a reconstructed sample by adding a residual sample to theprediction sample. In case a residual sample exists for the currentblock, the decoding apparatus may obtain information on residual for thecurrent block from the bitstream. The information on residual mayinclude a transform coefficient related to the residual sample. Thedecoding apparatus may derive the residual sample (or residual samplearray) for the current block based on the residual information. Thedecoding apparatus may generate a reconstructed sample based on theprediction sample and the residual sample, and the decoding apparatusmay derive a reconstructed block or a reconstructed picture based on thereconstructed sample. Afterwards, as described above, in order toenhance subjective/objective picture quality as needed, the decodingapparatus may apply in-loop filtering procedures, such as de-blockingfiltering and/or SAO procedure(s), to the reconstructed picture.

FIG. 19 illustrates a general view of a decoding apparatus performing animage decoding method according to the present disclosure. The methoddisclosed in FIG. 18 may be performed by the decoding apparatusdisclosed in FIG. 19. More specifically, for example, an entropy decoderof the decoding apparatus of FIG. 19 may perform step S1800 of FIG. 18,a predictor of the decoding apparatus of FIG. 19 may perform steps S1810to S1850 of FIG. 18, and an adder of the decoding apparatus of FIG. 19may perform step S1860. Additionally, although it is not shown in thedrawing, a process of obtaining information on a residual for thecurrent block through a bitstream may be performed by an entropy decoderof the decoding apparatus of FIG. 19, and a process of deriving theresidual sample for the current block based on the residual informationmay be performed by an inverse transformer of the decoding apparatus ofFIG. 19.

According to the above-described present disclosure, efficiency in imagecoding based on affine motion prediction may be enhanced.

According to the above-described present disclosure, when deriving anaffine MVP candidate list, neighboring blocks are divided into a leftblock group and a top block group, and an affine MVP candidate list maybe constructed by deriving MVP candidates from each block group. Thus,the complexity in the process of constructing the affine MVP candidatelist may be reduced, and the coding efficiency may be enhanced.

In the above embodiments, although the methods have been described basedon the flowcharts using a series of the steps or blocks, the presentdisclosure is not limited to the sequence of the steps, and some of thesteps may be performed at different sequences from the remaining stepsor may be performed simultaneously with the remaining steps.Furthermore, those skilled in the art will understand that the stepsshown in the flowcharts are not exclusive and may include other steps orone or more steps of the flowcharts may be deleted without affecting thescope of the present disclosure.

The embodiments described in this document may be implemented andperformed on a processor, a microprocessor, a controller, or a chip. Forexample, the functional units shown in each drawing may be implementedand performed on a computer, processor, microprocessor, controller, orchip. In this case, information (e.g., information on instructions) oran algorithm for implementation may be stored in a digital storagemedium.

In addition, the decoding apparatus and the encoding apparatus to whichthe present disclosure is applied can be applied to multimediacommunication devices such as a multimedia broadcasting transmitting andreceiving device, a mobile communication terminal, a home cinema videodevice, a digital cinema video device, a surveillance camera, a videochatting device, (3D) video devices, video telephony video devices, andmedical video devices, and the like, which may be included in, forexample, a storage medium, a camcorder, a video on demand (VoD) serviceprovision device, an Over the top video (OTT video), an Internetstreamlining service providing device, a 3D video device, a video calldevice, a transportation means terminal (e.g., vehicle terminal,airplane terminal, ship terminal, and so on) and may be used to processvideo signals or data signals. For example, the over the top video (OTTvideo) device may include a game console, a Blu-ray player, an Internetaccess TV, a home theater system, a smartphone, a tablet PC, a DigitalVideo Recorder (DVR).

Further, the processing method to which the present disclosure isapplied may be produced in the form of a computer-executed program, andmay be stored in a computer-readable recording medium. The multimediadata having the data structure according to the present disclosure canalso be stored in a computer-readable recording medium. Thecomputer-readable recording medium includes all kinds of storage devicesand distributed storage devices in which computer-readable data isstored. The computer-readable recording medium may be, for example, aBlu-ray Disc (BD), a Universal Serial Bus (USB), a ROM, a PROM, anEPROM, an EEPROM, a RAM, a CD-ROM, electromagnetic tape, a floppy disk,and optical data storage devices. In addition, the computer-readablerecording medium includes media implemented in the form of a carrierwave (for example, transmission over the Internet). In addition, the bitstream generated by the encoding method can be stored in acomputer-readable recording medium or transmitted over a wired orwireless communication network.

Further, an embodiment of the present disclosure may be implemented as acomputer program product by program code, and the program code may beexecuted in a computer according to an embodiment of the presentdisclosure. The program code may be stored on a carrier readable by acomputer.

FIG. 20 illustrates a content streaming system structure to which thepresent disclosure is applied.

A content streaming system to which the present disclosure is appliedmay include an encoding server, a streaming server, a web server, amedia storage, a user device, and a multimedia input device.

The encoding server compresses content input from multimedia inputdevices, such as smartphones, cameras, camcorders, and so on, intodigital data to generate a bitstream and transmit the bitstream to thestreaming server. As another example, when multimedia input devices,such as smartphones, cameras, camcorders, and so on, directly generate abitstream, the encoding server may be omitted.

The bitstream may be generated by an encoding method or a bitstreamgenerating method to which the present disclosure is applied, and thestreaming server may temporarily store the bitstream in the process oftransmitting or receiving the bitstream.

The streaming server transmits multimedia data to the user device basedon a user request through the web server, and the web server serves asan intermediary for informing the user of what services are provided.When a user requests a desired service from the web server, the webserver delivers it to a streaming server, and the streaming servertransmits multimedia data to the user. Here, the content streamingsystem may include a separate control server, and, in this case, thecontrol server controls a command/response between devices in thecontent streaming system.

The streaming server may receive content from a media repository and/oran encoding server. For example, when content is received from theencoding server, the content may be received in real time. In this case,in order to provide a smooth streaming service, the streaming server maystore the bitstream for a predetermined time.

Examples of the user device include a mobile phone, a smartphone, alaptop computer, a digital broadcasting terminal, a personal digitalassistant (PDA), a portable multimedia player (PMP), a navigationdevice, and a slate PC, a tablet PC, ultrabook, a wearable device (e.g.,smartwatch, glass glasses, head mounted display), a digital TV, adesktop computer, a digital signage, and so on. Each server in thecontent streaming system may operate as a distributed server, and inthis case, data received from each server may be processed in adistributed manner.

What is claimed is:
 1. A video decoding method performed by a decodingapparatus, comprising: obtaining motion prediction information for acurrent block from a bitstream; constructing an affine motion vectorpredictor (MVP) candidate list including affine MVP candidates for thecurrent block; deriving control point motion vector predictors (CPMVPs)for control points (CPs) of the current block based on CPMVP candidatesof one of the affine MVP candidates included in the affine MVP candidatelist; deriving control point motion vector differences (CPMVDs) for theCPs of the current block based on the motion prediction information;deriving control point motion vectors (CPMVs) for the CPs of the currentblock based on the CPMVPs and the CPMVDs; deriving prediction samplesfor the current block based on the CPMVs; and generating a reconstructedpicture for the current block based on the derived prediction samples,wherein the affine MVP candidates include a first affine MVP candidateand a second affine MVP candidate, wherein CPMVs derived based on anaffine motion model of a first block in a left block group including abottom-left corner neighboring block and a left neighboring block arederived as CPMVP candidates of the first affine MVP candidate, whereinthe first block is coded with the affine motion model of the first blockand a reference picture of the first block is same as a referencepicture of the current block, wherein CPMVs derived based on an affinemotion model of a second block in a top block group including atop-right corner neighboring block, a top neighboring block, and atop-left corner neighboring block are derived as CPMVP candidates of thesecond affine MVP candidate, and wherein the second block is coded withthe affine motion model of the second block and a reference picture ofthe second block is same as the reference picture of the current block.2. The method of claim 1, wherein the first block is a first blockverified to be satisfying a condition when checking neighboring blockswithin the left block group according to a specific order.
 3. The methodof claim 1, wherein the second block is a first block verified to besatisfying a condition when checking neighboring blocks within the topblock group according to a specific order.
 4. The method of claim 3,wherein the specific order is an order starting from the top neighboringblock to the top-right corner neighboring block and the top-left cornerneighboring block.
 5. The method of claim 1, wherein the motionprediction information includes an affine MVP candidate index for thecurrent block, and wherein the CPMVPs for the CPs of the current blockare derived based on CPMVP candidates of an affine MVP candidate beingindicated by the affine MVP candidate index.
 6. The method of claim 1,wherein the step of constructing an affine motion vector predictor (MVP)candidate list, comprises: dividing motion vectors of neighboring blocksof the current block into a first group, a second group, and a thirdgroup; and deriving a CPMVP candidate for CP0 of the current block fromthe first group, a CPMVP candidate for CP1 of the current block from thesecond group, and a CPMVP candidate for CP2 of the current block fromthe third group, and deriving a constructed affine MVP candidateincluding CPMVP candidates for the CPs.
 7. The method of claim 6,wherein the neighboring blocks include neighboring block A, neighboringblock B, neighboring block C, neighboring block D, neighboring block E,neighboring block F, and neighboring block G, and wherein, in case asize of the current block is W×H, and an x component and a y componentof a top-left sample position of the current block are 0, theneighboring block A is a block including a sample of coordinates (−1,−1), the neighboring block B is a block including a sample ofcoordinates (0, −1), the neighboring block C is a block including asample of coordinates (−1, 0), the neighboring block D is a blockincluding a sample of coordinates (W−1, −1), the neighboring block E isa block including a sample of coordinates (W, −1), the neighboring blockF is a block including a sample of coordinates (−1, H−1), and theneighboring block G is a block including a sample of coordinates (−1,H).
 8. The method of claim 7, wherein the first group includes a motionvector of the neighboring block A, a motion vector of the neighboringblock B, and a motion vector of the neighboring block C, wherein thesecond group includes a motion vector of the neighboring block D and amotion vector of the neighboring block E, and wherein the third groupincludes a motion vector of the neighboring block F and a motion vectorof the neighboring block G.
 9. The method of claim 8, wherein the CPMVPfor CP0 is a motion vector having a reference picture first verified tobe the same as a reference picture of the current block, when checkingmotion vectors within the first group according to a specific order,wherein the specific order is an order starting from the neighboringblock A to the neighboring block B and the neighboring block C.
 10. Themethod of claim 8, wherein the CPMVP for CP1 is a motion vector having areference picture first verified to be the same as a reference pictureof the current block, when checking motion vectors within the secondgroup according to a specific order, wherein the specific order is anorder starting from the neighboring block D to the neighboring block E.11. The method of claim 8, wherein the CPMVP for CP2 is a motion vectorhaving a reference picture first verified to be the same as a referencepicture of the current block, when checking motion vectors within thethird group according to a specific order, wherein the specific order isan order starting from the neighboring block F to the neighboring blockG.
 12. A video encoding method performed by an encoding apparatus,comprising: constructing an affine motion vector predictor (MVP)candidate list including affine MVP candidates for a current block;deriving control point motion vector predictors (CPMVPs) for controlpoints (CPs) of the current block based on CPMVP candidates of one ofthe affine MVP candidates included in the affine MVP candidate list;deriving control point motion vectors (CPMVs) for the CPs of the currentblock; deriving control point motion vector differences (CPMVDs) for theCPs of the current block based on the CPMVPs and the CPMVs; and encodingmotion prediction information including information on the CPMVDs,wherein the affine MVP candidates include a first affine MVP candidateand a second affine MVP candidate, wherein CPMVs derived based on anaffine motion model of a first block in a left block group including abottom-left corner neighboring block and a left neighboring block arederived as CPMVP candidates of the first affine MVP candidate, whereinthe first block is coded with the affine motion model of the first blockand a reference picture of the first block is same as a referencepicture of the current block, wherein CPMVs derived based on an affinemotion model of a second block in a top block group including atop-right corner neighboring block, a top neighboring block, and atop-left corner neighboring block are derived as CPMVP candidates of thesecond affine MVP candidate, and wherein the second block is coded withthe affine motion model of the second block and a reference picture ofthe second block is same as the reference picture of the current block.13. The method of claim 12, wherein the first block is a first blockverified to be satisfying a condition when checking neighboring blockswithin the left block group according to a specific order.
 14. Themethod of claim 12, wherein the second block is a first block verifiedto be satisfying a condition when checking neighboring blocks within thetop block group according to a specific order.
 15. The method of claim14, wherein the specific order is an order starting from the topneighboring block to the top-right corner neighboring block and thetop-left corner neighboring block.
 16. A non-transitorycomputer-readable storage medium storing a bitstream, the bitstream,when executed, causing a decoding apparatus to perform the followingsteps: obtaining motion prediction information for a current block fromthe bitstream; constructing an affine motion vector predictor (MVP)candidate list including affine MVP candidates for the current block;deriving control point motion vector predictors (CPMVPs) for controlpoints (CPs) of the current block based on CPMVP candidates of one ofthe affine MVP candidates included in the affine MVP candidate list;deriving control point motion vector differences (CPMVDs) for the CPs ofthe current block based on the motion prediction information; derivingcontrol point motion vectors (CPMVs) for the CPs of the current blockbased on the CPMVPs and the CPMVDs; deriving prediction samples for thecurrent block based on the CPMVs; and generating a reconstructed picturefor the current block based on the derived prediction samples, whereinthe affine MVP candidates include a first affine MVP candidate and asecond affine MVP candidate, wherein CPMVs derived based on an affinemotion model of a first block in a left block group including abottom-left corner neighboring block and a left neighboring block arederived as CPMVP candidates of the first affine MVP candidate, whereinthe first block is coded with the affine motion model of the first blockand a reference picture of the first block is same as a referencepicture of the current block, wherein CPMVs derived based on an affinemotion model of a second block in a top block group including atop-right corner neighboring block, a top neighboring block, and atop-left corner neighboring block are derived as CPMVP candidates of thefirst affine MVP candidate, and wherein the second block is coded withthe affine motion model of the second block and a reference picture ofthe second block is same as the reference picture of the current block.